Feedback mechanisms between T helper cells and macrophages
in the determination of the immune response
Ruth Lev Bar-Or
*Department of Applied Mathematics and Computer Science, The Weizmann Institute of Science, P.O. Box 26, 76100 Rehovot, Israel
Received 5 February 1999; received in revised form 11 June 1999; accepted 24 August 1999
Abstract
The interactions between macrophages and T helper (Th) cells are a complex interplay of positive and negative signals. Some of the mathematical models of interactions between T helpers have indeed taken the in¯uence of macrophages into account. In this work the macrophage is not considered as an extrinsic agent, that is duly directed by the T cells to be cytotoxic, nor is there consideration of T helper cell populations that are dominantly regulated by extrinsic properties of antigens per se, or by certain classes of presenting cells that preferentially select certain classes of lymphocytes or bias their commitment. Rather, a simpli®ed model of feedback loops between Th cells and macrophages is formulated and analyzed. It is suggested how the mutual in¯uence between Th and macrophages can determine the cytokine secretion pattern of these populations. The model provides a feedback scenario to account for experimental ®ndings concerning reversal in the dominance of a speci®c cytokine pro®le in the course of some infections. A possible scenario accounting for the dierence between the stability of Th1 and Th2 cytokine pattern is put forward. The model suggests explanations for the variability in the outcome of the immune response according to dif-ferent body compartments. A rationale is presented that accounts for paradoxical ®ndings indicating that Th1 cytokines are sometimes responsible for the downregulation of a Th1 dominated response. Ó 2000 Elsevier Science Inc. All rights reserved.
Keywords:Mathematical model; Th1/Th2; Feedback loops; Macrophages; Cytokines
www.elsevier.com/locate/mbs
*Tel.: +972-8 934 2390; fax: +972-8 934 4122.
E-mail address:ruthy@wisorg.weizmann.ac.il (R. Lev Bar-Or).
1. Introduction
Since the discovery that CD4+ T cell subsets secrete dierent patterns of cytokines [1±3] sub-stantial evidence has accumulated showing the importance of the T helper 1 (Th1) and Th2 subsets in the success or failure of immune responses. Th1 cells, which secrete mainly IL-2 and IFN-c, promote immunity to infections by intracellular bacteria, protozoa and viruses [4,5]. Th2 cells, secreting mainly IL-4, IL-5, IL-6, IL-10 and IL-13 control responses against extracellular pathogens and are involved in the pathogenesis of allergic reactions [6]. Both Th1 and Th2 cells appear able to cross-suppress each other via the cytokines they secrete, and this provides an explanation for the inverse relationship observed between Th1 and Th2-directed responses.
It is important to keep in mind though, that the Th1/Th2 dichotomy is a simpli®ed version of reality. Additional CD4+ T cell phenotypes have been discussed, such as the Th0 phenotype that secretes both Th1 and Th2 cytokines, indicating that these two classes do not represent two absolute and mutually exclusive states [7,8]. Furthermore, the terminology referring to `Th1' or `Th2' responses is misleading, since many of the Th1/Th2 cytokines are also produced by other cell types. For example, IFN-cis also secreted by natural killer cells, CD8+ cells, andcdcells; IL-10 is also produced by macrophages and mast cells [9±11]. Finally, recent evidence indicates that CD8+ T cells cannot only secrete Th1 but also Th2-like cytokine patterns [12±16]. In spite of the simpli®cation imbedded in the Th1/Th2 model, evidence shows that the state of health ± the ability to ®ght infectious agents eectively, and not to succumb to autoimmune diseases ± often depends on which pattern (Th1 or Th2) overshadows which, i.e. whether an appropriate or an inappropriate immune response has been achieved.
In the framework of orchestrating an immune response, the pattern of cytokines produced by dierent subsets of T lymphocytes, are used as communication signals with many other types of cells, such as B cells and macrophages [4]. Macrophages are central in host immunity to patho-genic agents. They are the major eector cells that contain and kill intracellular pathogens. Op-timal macrophage function is dependent on activation of the cells by a variety of extracellular (as well as intracellular) stimuli, most prominently T-cell-derived lymphokines. Such cytokine pro-duction requires antigen recognition by T cells, a process in which macrophages are also involved as antigen processing and presenting cells [17].
In this work an attempt is made to take a further step in the understanding of the rich, complex, bidirectional interactions between macrophages and T cells by formulating a simpli®ed model of feedback loops between Th cells and macrophages. It is hoped thereby to gain some insights into factors that determine the success or failure of immune responses by engendering a choice between a dominant Th1 or a dominant Th2 secretion pattern. An understanding of these factors may provide a means for therapeutic intervention in situations wherein the immune response is not ecient or even deleterious to the host.
The macrophage is not considered as an extrinsic agent, that is duly directed by the T cells to be cytotoxic, nor is there consideration of T helper cell populations that are dominantly regulated by extrinsic properties of antigens per se, or by certain classes of APC that preferentially select certain classes of lymphocytes or bias the commitment of uncommitted lymphocytes (reviewed in [23]). Rather, a key question addressed here is how does feedback between Th cells and macro-phages allow these populations the possibility of in¯uencing their own fate.
The following are some phenomena that will be addressed. (i) It is believed that for some in-fections, dierent eector functions may be appropriate at dierent stages of infection, such as in a mouse malaria model in which Th1 and Th2 responses may be important early and late in in-fection, respectively [24,25]. (ii) Sometimes, too much of a Th1 cytokine impairs a strong Th1 response [26,27]. (iii) The Th2 cytokine pattern appears more stable than the Th1 pattern [28±30]. (iv) The in¯uence of the location of the immune response (in dierent tissues and body cavities) on the type of dominance (Th1 or Th2) of the resulting response. (See for example [31±33].)
2. Biological background
T cells and macrophages use cytokines to communicate with each other and among themselves, and alter each otherÕs behavior. Table 1 is an attempt to provide a crude summary of how cy-tokines produced by Th cells aect several functions of macrophages (cytokine secretion and antigen presentation), and also in¯uence the behavior of the Th population itself, and of how macrophage-derived cytokines modulate cytokine secretion by Th cells and by macrophages. The next four paragraphs recount the main sources of information.
Cytokines produced by Th cells upon speci®c antigen recognition, modulate diverse eector functions of macrophages [31]. First, T helper cells aect the ability of macrophages to secrete several cytokines. The Th1 cytokine IFN-c(also reported to be produced ± by a lesser extent ± by macrophages, [34]) enhances the macrophagesÕsecretion of products such as nitric oxide (NO), which plays an important role in the antimicrobial activities of macrophages [35]. IFN-c also enhances the secretion of several cytokines, such as TNF-a, which possess antiviral activity [36], and IL-12, which enhances the cellsÕ cytotoxic activity (reviewed in [5]). IFN-c also inhibits macrophage secretion of IL-10 [37]. Th1 cytokines TNF-a (also produced by macrophages) and TNF-b induce the production of several macrophage products such as GM±CSF, which shows microbicidal activity, and NO (reviewed in [38]). Th product GM±CSF, (also produced by macrophages) induces TNF-asecretion [38,39].
Table 1
Eects of Th and macrophage-derived cytokines
Cytokine Produced by (among other
cells)
Enhances production of Decreases production of Eect on MHCII Grouped under (model)
IFN-c Th; (macrophages) NO, TNF-a, IL-12 IL-10. Suppresses
prolif-eration of Th2 cells
GM±CSF Th; macrophages TNF-a CT
1;C M 1
IL-12 Macrophages IFN-c, TNF-a, GM±CSF
dierentiation of Th1
IL-18 Macrophages IFN-c, GM±CSF IL-10 CM
(also produced by macrophages) which inhibits the secretion of proin¯ammatory cytokines such as TNF-a, GM±CSF, IL-12, and also inhibits nitric oxide production [1,5,41]. The Th2 cytokine TGF-b (also produced by macrophages) inhibits the production by macrophages of TNF-a, of reactive oxygen products, of IL-12, and other proin¯ammatory cytokines (reviewed in [5,42]). TGF-bis also known to promote IL-10 production by macrophages [43]. The Th2 cytokine IL-13 shows a strong inhibitory activity on in¯ammatory cytokine production, such as TNF-a and GM±CSF, it decreases the production of nitric oxide in murine macrophages [44], and is a neg-ative regulator of the production of IL-12 [5].
T helper cells not only aect the macrophagesÕsecretion ability of cytokines, but also in¯uence antigen presentation, by regulating the expression of MHC II molecules±molecules whose binding to the processed antigen is essential in order for the helper cells to recognize it [45]. IFN-c, TNF, GM±CSF, IL-4 and IL-13 upregulate MHC II expression ([46] for IFN-c; [47] for TNF-a; [48] for GMC±CSF; [49] for IL-4; [50] for IL-13). In contrast, IL-10 and TGF-bwere shown to inhibit the expression of class II molecules on macrophages ([51] for IL-10; [52] and [42] for TGF-b).
Not only does the type of cytokines produced by activated T cells in the process of antigen presentation in¯uence the speci®c pattern of macrophage functions, but the eector functions of the Th cells themselves are modulated in turn by the eects of macrophages. TNF-aenhances the production of GM±CSF (by both T cells and macrophages), and IFN-c[38]. Macrophage product IL-12 induces the production of IFN-c, TNF-aand GM±CSF. IL-12 has inhibitory eects on the generation of Th2 cells (reviewed in [5,53]), and according to novel data on IL-4 and IL-10 se-cretion [54,55]. GM±CSF induces TNF-a secretion (by T cells and macrophages). IL-18, a new cytokine found to be produced by macrophages, enhances IFN-cand GM±CSF production by Th cells, and inhibits the secretion of IL-10 by Th cells, and macrophages [56]. Several macrophage products are known to downregulate cytotoxic functions of T cells (and also macrophages). IL-10 inhibits the secretion of TNF-a, GM±CSF, and IFN-c by T cells [37]. The macrophage product prostaglandin E2 (PGE2) inhibits the secretion of TNF-a and IL-12, and downregulates MHC class II expression [57±59]. Finally, TGF-b inhibits the production of TNF-a and enhances the production of IL-10.
3. The model
Accumulating research on cytokines paint a picture that seems hopelessly complex. Pleotropism and redundancy abound. As sketched above, a single cytokine can interact with more than one type of cell, a single cytokine can have multiple biological activities, a single cell can interact with more than one cytokine, many cytokines have overlapping activities, and a single cytokine may induce or inhibit the expression of a gene encoding another cytokine. Moreover, during the last decade, 5±10 new cytokines have been cloned every year [60]. Today more than 100 cytokines have been cloned. Hundreds more probably exist.
cytokines such as IFN-c, TNF-a and -b, and GM±CSF. Cytokine pro®le ÔT-Type2Õ lumps to-gether T helper cell-derived cytokines such as IL-4, IL-10, IL-13, and TGF-b.ÔM-Type1ÕandÔ M-Type2Õ denote cytokines secreted by macrophages, which as seen in Section 1, have extensive overlap with T helper-derived cytokines. The category ÔM-Type1Õ contains macrophage-derived in¯ammatory cytokines such as TNF-a, IL-12, GM±CSF, IL-18, NO, and other antimicrobicidal products. ÔM-Type2Õ bundles macrophage-derived anti-in¯ammatory cytokines such as IL-10, PGE-2, and TGF-b.
The model envisions a population comprised by nT T helper cells, which may have dierent
antigen speci®cities, and nM macrophages that are active within a speci®c target area, such that
every cell can aect all others. Any given Th celli i1;. . .;nTcan secrete certain amounts of T
-Type1 and T-Type2 cytokines, denoted respectively by C1Ti and C Ti
2 . Any given macrophage
i i1;. . .;nM can secrete certain amounts of M-Type1 and M-Type2 cytokines, denoted
re-spectively by C1Mi and C2Mi. The parameters chosen to characterize each of the four cytokine pattern types are taken to be qualitative averages of the characteristics of the array of cytokines that constitute each category.
C1T and C2T denote the arithmetic average of secreted cytokine amounts CTi1 and C Ti
2 over the
whole T cell population. Similarly, C1M and C2M denote the mean secretion levels of Type-1 and Type-2 cytokines over the macrophage population.
An important point taken into consideration is that, as mentioned in Section 1, it is an over-simpli®cation to say that T helper cells have totally distinct patterns of cytokine secretion. There is evidence indicating the possibility that each individual T cell is capable of producing the entire array of cytokines [40]. This picture promoted the idea to leave aside the destiny of the T helper cells themselves, and to focus on their products (in the form of cytokines). Thus it is not discarded that some T helper cells secrete both cytokine pro®les simultaneously.
The model is illustrated in Fig. 1. Fig. 1(a) depicts a schematic representation of the interac-tion mechanisms between T helpers and macrophages, and within each group, that are taken into account in the model. Fig. 1(b) represents the modeling of the eect that the multiple signals have on the secretion of each cytokine pro®le CT1, C2T, C1M and C2M. The basic phenome-nology that remains after lumping numerous cytokines into four pro®les is (i) cross inhibition between Type-1 and Type-2 secretion pro®les of macrophages and T helpers, (ii) excitation among each of both Type-1 and Type-2 subclasses, and (iii) upregulation and down(up)regulation of the macrophagesÕpresentation ability by Type-1 and Type-2 (Th and macrophage-derived) secretion pro®les.
The following equations for the responsesCT
1,C2T are assumed:
In (1) the coecients d1,d2 specify characteristic decay times.hT1 tand h T
2 tdesignate the total
input aecting the secretion ofCT
1 and CT2 at timet.gis a sigmoidal gain function of the inputh,
chosen for the sake of simplicity to be
g hT 1
2 1
ÿ
tanhÿhTÿh
: 2
constitute a general framework that embodies the idea of excitation, via saturation and threshold. The nature of the excitation and its related terms are discussed below.
The terms that contribute to the inputshT
The ®rst term in both sums, hT non-specific, re¯ects the contribution of non-speci®c Th and macrophageÕs cytokines to the input and is given by
hT1 non-specific ÿh
Underlying this choice of expression are the observations presented in Section 1 (and summarized in Table 1), according to which Th cytokine pro®leCT
1 and macrophage cytokine pro®leC1Mhave
± on the average ± a positive eect on CT1 secretion (and a negative eect onC2T production). In contrast, Th cytokine pro®le CT
2 and macrophage cytokine pro®leC2Mhave ± on the average ± a
negative eect on CT1 secretion (and a positive eect on CT2 secretion). The positive coecientsJ
denote the sensitivity of CT
1 (orC2T) secretion to Th-derived cytokines, and JM denote their
sen-sitivity to macrophage-derived cytokines.
The second part of the input term aecting the secretion of cytokine patternsCT1 andC2T relies on speci®c interaction mechanisms between macrophages and Th cells, namely on antigen pre-sentation by the former. This interaction is implemented as follows:
hT1 presentationb1Ag ap1C1Tap2C2Tap
In Eq. (5),Agrepresents the amount of presented antigen andb1 andb2 denote the sensitivity of
C1TandCT2 secretion to the antigen. In biological terms,b1 andb2 can quantitate memories of past antigen exposure. A largebre¯ects a high overall memory of the Th population of the antigen. If, for instance,b1>b2, then a given antigen will induce a larger impact on theC1Tcytokine secretion pattern than on the CT
2 cytokine secretion pattern. In addition to the antigen amount and the
sensitivity to it, the input due to macrophage presentation is also determined by the ability of the macrophages to exert their presentation functions. The term in brackets represents the presen-tation ability of the macrophage, a function of positive and negative signals exchanged between the macrophages and the Th cells, and between the macrophages themselves. The sensitivity coecients [ap] provide a measure of how these signals aect the antigen presentation ability. As summarized in Table 1, the cytokines categorized asC1T and C1M, have on the average a positive eect on the macrophagesÕpresentation ability, through their eect on MHC class II upregulation. This implies positive ap1 and ap1M. In contrast, it is not clear whether ap2 and apM2 should be
positive or negative: Type-2 Th-derived cytokines IL-4 and IL-13 have positive eects on MHC class II expression, whereas Type-2 Th and macrophage-derived cytokines IL-10, PGE-2 and TGF-bare known to downregulate the expression of this molecule. Both possibilities are therefore implemented and tested.
The dynamics of the macrophagesÕcytokine secretion patterns,C1M andC2M, are governed by dÿC1M=dt ÿdM
hM1 K1C1TÿK2CT2 K
In Eq. (5), the dependence of the macrophagesÕ presentation ability on non-speci®c Th and macrophage-derived cytokines is implemented. Eq. (7) embodies the dependence of the macro-phagesÕsecretatory function, on the above cytokines. As seen in Section 1, and reviewed in Table 1, Th cytokines classi®ed under cytokine pro®le C1T and macrophage cytokine pro®le CM1 have a positive eect onCM
1 secretion (and a negative eect onC2Mproduction). In contrast, Th cytokine
pro®le C2T and macrophage cytokine pro®le C2M have a negative eect on C1M secretion (and a positive eect onCM
2 production). The positive coecientsKdenote the sensitivity ofC1M(orCM2 )
secretion to Th cytokines while theKM denote the sensitivity to macrophage-derived cytokines. The sensitivitiesJ,JM(Eq. (4)) andK,KM, can be biologically interpreted as the quantity and or/
quality of receptors for the dierent cytokines. As an example, the major structural component of the bacterial outer membrane, namely lipopolysaccharide (LPS), is known to induce changes in responsiveness to cytokines, by mediating cytokine receptor expression in macrophages [63]. In the terms of the model, this eect is translated as a modi®cation of the Kvalues.
It is important to stress that the array of sensitivitiesJ1, J2, K1,K2 can also be interpreted as a
measure of the number of Th cells. Similarly, J1M, J2M, K1M, and K2M can re¯ect the number of macrophages. The reason is as follows. ConsidernTTh cells, andnMmacrophages. According to a
cellular version of Eq. (4), the non-speci®c part of the input aecting the secretion capacity of cytokineC1Ti of cell i is as arithmetic averages of the cytokines secreted by thenTTh cells andnMmacrophages. ThusJ1in
Eq. (4) equals (j1nT),J2equals (j2nT),J1Mequals (j
In the model, the question of early events that induce initial dierentiation from a naive cell is left out. Here, established T helper cells and macrophages already exist, and it is investigated how this system evolves after an initial event:ÔsomethingÕinduces initial levels of activity among the T helper and macrophage populations.
As seen before, the groups denoted asCT
1 andC1Mshare extensive overlap. TNF and GM±CSF,
for instance, belong to both classes and have similar or complementary functions in the pro-motion of in¯ammatory and other responses classically categorized and Th1. Similarly, cytokines classi®ed asCT2 andC2Mare both associated with functions that are classically categorized as Th2. The choice of a particular immune response can thus be viewed as between one in whichC1T and
CM
1 secretion pro®les predominate, and one in whichCT2 and CM2 pro®les predominate. Denoting
by `Type-1 pro®le' the sumC1 12 CT1 C M
1 and by `Type-2 pro®le' the sumC2 12 CT2 C M 2 , I
simulations in cases for which formal explicit analysis proved too dicult. Examination of sample sections of the four-dimensional space and time varying plots were used to assert stability.
4. Results
The dependence of the steady-state solution forC1 and C2 on the dierent parameter sets was
examined graphically by time varying plots and by sample sections of the four-dimensional space. Fig. 2 is a projection of this space that helps us to visualize the dependence of the number of steady-state solutions on the sensitivity valuesJs andKs (Eqs. (4) and (7)). In the case shown in
Fig. 2. A sample section of the four-dimensional space that illustrates the in¯uence of the sensitivities (J1;J2;JM 1 ;J2M; Eq. (4)) on the steady-state solutions. From numerical simulations, we obtain that for small values of bothJ1JM
1 and
J2JM
2 , a unique stable state is possible. Type-1 dominance, i.e.,C1>C2whenJ1J1M>J2J2M; Type-2 dominance whenJ1JM
1 >J2J M
2 : (a) Steady-state values of T cell and macrophage-derived cytokines,C T 1;C
T 2;C
M 1 andC
M 2 . (b) Instead of four cytokine pro®les, only two classes, i.e., Type-1 (C1) and Type-2 (C2) are used to characterize the be-havior of the system. (c) For larger sensitivity values, a bistability region exists for which either a Type-1 or a Type-2 dominated response is possible.b1b21; Ag1; ap's0:05; here and throughout the following ®gures I take d's1; h0. In (a) J1JM
1 K1K M
1 0:4; J2J M
2 K2K M
2 0:5. In (b) J1J M
1 K1K M
1 0:6; J2 J2MK2K
Fig. 2(a), only one stable steady state is attainable. This is a Type-2 dominated stable steady state (C1 >C2) if the average sensitivities to cytokine pro®lesC1T andC
M
1 , are less than the sensitivities
to cytokine pro®lesCT
2 andC2M, i.e. ifJ1J1M<J2J2M. A Type-1 dominated stable steady state
appears, in contrast, when J1J1M>J2J2M (not shown). The larger the gap between the
sen-sitivity values, the stronger the dominance of the prevailing type. For larger sensen-sitivity values however, a bistability region exists for which either a Type-1 or a Type-2 dominated response is possible, as shown in Fig. 2(b). Here, althoughJ1J1M<J2J2Mthen unlike the previous case, a
Type-1 dominated response is also possible. The initial levels of activation ± or initial conditions ± induced in the Th population and or/the macrophage population determine the outcome of the response, selecting between these two possible steady states according to their domains of at-traction.
The results presented above stem from the choice of the sigmoidal gain function of the input, made in Eq. (2). From Eq. (4) it can be seen that for small values ofJs that give rise ± all else being equal ± to small inputs hT1;hT2, the gain function can be approximated by a linear function, al-lowing only one stable steady state solution. The approximation is no longer valid for the average of larger values ofJs, due to the choice of the non-linear gain function. Larger input values give rise to three steady-state solutions (two stable and one unstable), provided that the gap between the cross sensitivities, i.e.J1J1M and J2J2M is not too large.
In the following, I show representative cases of biologically interesting behavior of the model, and therefore sample parameter sets were chosen for presentation. For a more comprehensive analysis of the systemÕs behavior as a function of its parameter values, see [64].
Note that the results of the model predict long-term coexistence of both Type-1 and Type-2 responses in the same site ± one of them overshadowing the other. Such coexistence has been observed by [33,65±69]. We note that in the theoretical model formulated in [18], the resulting picture is of exclusive dominance of either the Th1 or the Th2 secretion pattern. In the present model, certain parameter values can give rise to a response strongly dominated by one secretion type. In this case, the dierence between the outcomes of the two models, namely between slight and no coexistence is immaterial. However, the pictures diverge substantially in the cases of re-sponses characterized by almost equal Type-1 and Type-2 components, obtained in the present model for certain parameter values. The results of coexistence can be traced to the fact that in the gain function of the input to T cells and macrophages, i.e., Eq. (2), there is a small constitutive background excitability. That is, a small constant source of cytokines ± secreted by other cells active in the environment, such as other tissue cells and natural killers ± is supplied even in the absence of Th cells and macrophages.
4.1. Auto-reinforcement of response via macrophage±Th cell interactions
Suppose that we have a situation wherein a certain set of conditions led to a Type-1 dominated response. Let us now ponder ways to bring about a stronger Type-1 response, as depicted in Fig. 3(a). One way is to add (by arti®cial intervention in the lab) sucient amounts of Type-1 cyto-kines. This adds a constant positive term to thehnon-specific1 in Eqs. (4) and (7). However, adding copious amounts of cytokines can have unwelcome harmful eects on the host. Another way to accomplish the task is shown in Fig. 3(b). Reinforcement is obtained when the sensitivities of the secretion pro®les of the Th population to macrophage-derived cytokines, i.e.,JM
are increased with respect to their values in Fig. 3(a). The same eect is accomplished if the sensitivities of the macrophagesÕsecretion pro®les to Th-derived cytokines (i.e., K1 and K2) are
increased instead.
Fig. 3. Reinforcement of the response via macrophage±Th cells interactions. (a) A Type-1 dominated response is obtained. (b) Strengthening the sensitivities of the secretion pro®les of the Th population to macrophage-derived cy-tokines brings about a stronger Type-1 response. (c) Strengthening the sensitivity of the macrophagesÕpresentation ability to Type-1 cytokines reinforces the Type-1 response obtained in (a)b1b21; Ag1; ap's0:05; J1K1
KM
1 0:5; J2K2K M
2 0:4; J M 1 J
M
2 0:01, in (a); J M 1 J
M
Besides non-speci®c cytokine mediated interactions between macrophages and Th cells, inter-actions in the context of antigen-presentation (Eq. (5)) can also enhance the predominant re-sponse. One way to reinforce a, e.g., Type-1 dominated response via antigen presentation mechanisms is to increase the sensitivityb1 ± orÔmemoryÕ± of the Type-1 secretion pattern with respect to b2. Another way to accomplish the task is shown in Fig. 3(c). Even if the antigen amount is constant, reinforcement is obtained by increasing the sensitivity of the macrophagesÕ
presentation ability to Type-1 cytokines secreted by the Th population.
We thus conclude from the above that tightening the macrophages±Th cell feedback mecha-nisms (via non-speci®c cytokines or via antigen presentation) can reinforce a prevailing response.
4.2. Auto-inhibition of response via macrophage±Th cell interactions
4.2.1. Stability of Type-1 vs. Type-2 dominated responses
In the previous section, we discussed the possibility of reinforcement of the prevailing response through ± among other ways ± antigen presentation mechanisms. As presented below, stronger presentation can, under certain conditions, reverse the type of dominance. As exempli®ed in Fig. (4), in suitable circumstances, antigen presentation can shift equally Type-1 to Type-2 or Type-2 to Type-1.
Considerable experimental evidence seems to point to an asymmetry in switching from one to the opposite type of response. (Whether this phenomenon takes place on a cellular or a popu-lation level is a matter of debate. [30] supports the latter hypothesis.) A number of works report on the stability of Th2 cytokine pattern ± in contrast to Th1 [28±30].
Motivated by these ®nding, I attempted to envision scenarios in which, for instance, a Type-1 to Type-2 switch is easier to achieve than a switch in the opposite direction. The switching behavior shown in Fig. 4(a) and (b) takes place under the conditionb2>b1, i.e., a larger sensitivity of the
CT
2 cytokine pro®le to the presented antigen than the sensitivity of theC1T cytokine pro®le to the
antigen. Increasing presentation (by taking a larger amount of antigen) can cause a switch from a Type-1 to a Type-2 dominated state. However, this mechanism does not induce the opposite switch from a Type-2 to a Type-1 dominated state (not shown). Associating the sensitivitiesbwith concept of memory to antigen, we can conclude the following. If the system has a Type-2 memory to a speci®c antigen, but nonetheless has attained a Type-1 response, then it isÔeasierÕto switch to the opposite Type-2 pattern than to reverse an attained Type-2 response. Thus, according to the above scenario, the asymmetry in switching from one to the opposite type of response can be encoded in a dierence in sensitivities (or memories) of the Th-cytokine secretion patterns to the external stimulus.
A qualitatively dierent type of scenario that can account for the switching asymmetry is discussed next. Increasing the amount of presenting antigen can cause a switch from a Type-1 to a Type-2 dominated state (Fig. 5(a) and (b)), but not a switch in the reverse direction (not shown), when the macrophagesÕ presentation ability has a larger positive sensitivity to Th-derived CT
1 cytokines than to C2T cytokines, namely when ap1 >ap2. That this type of
asym-metry in the sensitivities [ap] may be biologically plausible was indicated before. It was seen in Section 1 and in Table 1 that cytokines characterized as CT
1 have on the average a positive
eect on the macrophagesÕ presentation ability, through their eect of MHC class II upregu-lation. With respect to CT
(IL-4 and IL-13) have positive eects on MHC class II expression, whereas others (IL-10, PGE-2 and TGF-b) downregulate the expression of this molecule. Thus, averaging these properties may result in ap1 >ap2.
Note that the second scenario proposed to account for the asymmetric reversibility diers qualitatively from the ®rst in that it does not rely on qualities of the Th-population with respect to an external stimulus. Rather, it is based on macrophage±Th cells interaction, speci®cally on the way that the Th population aects the macrophages to present antigen to them.
The issue of asymmetry is a particular aspect of a general phenomena of reversibility of re-sponses. Experimental evidence indicates that for the system to generate a stable secretion pattern (which may reinforce itself in the process) is not always the best strategy. It is believed that for some infections, dierent eector functions may be appropriate at dierent stages of infection. An example is a mouse malaria model in which Th1 and Th2 responses may be important early and late in infection, respectively [24,25]. Th1±Th2 switches also occur in the course of self-limiting auto-immune disease. In a study of experimental auto-immune myocarditis (EAM) the authors demonstrate that Th1 type cytokines are expressed in the in¯ammatory phase of EAM and are subsequently followed by the expression of Th2 type cytokines in the recovery phase [70]. By
Fig. 4. Stability of 1 vs. 2 responses. In (a)±(d), increasing the antigen amount can shift a 1 to a Type-2, or a Type-2 to a Type-1 dominated response. (a) A Type-1 response is attained. (b) Increasing the antigen amount causes a switch to a Type-2 dominated response. (c) A Type-2 dominated response is attained. (d) Increasing the antigen amount causes a switch to a Type-1 dominated response. In (a),Ag0:1; b10:2; b21; ap's0:05; J1J1M
K1KM
1 0:5; J2J M
2 K2K M
2 0:45;initial Type-1 conditions. In (b),Ag10, and remaining parameter val-ues as in (a). In (c), J1JM
1 K1K M
1 0:45; J2J M
2 K2K M
introducing macrophage±Th cell feedback mechanisms, we have thus obtained a possible scenario that can account for these phenomena.
4.2.2. Lots of Type-1 cytokine, little Type-1 response
According to seemingly paradoxical ®ndings, too much of a Type-1 cytokine, can shut down a Type-1 dominated response. For example, the proin¯ammatory Th1 cytokine IFN-cis thought to play a pivotal role in the pathology of EAE, which has been shown to be a Th1 mediated disease. In [27] it is found that in vivo inhibition of IFN-c enhances EAE, thus concluding that IFN-c
apparently acts in some manner to down-regulate disease. IFN-cis also known to be involved in combating tuberculosis. However, in [71] it is reported that incubation of human macrophages or monocytes with this lymphokine often causes increased proliferation of the pathogen.
Apparently, NO has a dual role in in¯ammation as well. On the one hand, NO is cytotoxic, and can contribute to tissue destruction in in¯ammatory responses. On the other hand, NO produced by macrophages seems to act as an immune-suppressive agent. In [26] it is reported that when rats were treated with inhibitors of NO synthase, this resulted in a marked aggravation of clinical signs
Fig. 5. Eect of the sensitivity of the macrophagesÕpresentation ability [ap] to cytokines, on the switching asymmetry. (a) A Type-1 dominated response is attained. (b) A switch is obtained for larger antigen amount. In (a) b1b21; J1J1MK1K
M
1 0:6; J2J M
2 K2K M
2 0:7; ap2ap M
2 0; ap1ap M
of EAE. Thus they conclude that therapeutic intervention of EAE based on NO modulation should perhaps be aimed at increasing rather than inhibiting local NO production.
In order to account for these confusing observations, [35] proposes that NO might somehow inhibit the production of IFN-c, and this in turn inhibits its own synthesis. From the model, I propose a scenario relying on antigen-presentation mechanisms which, as discussed in the pre-vious section, can induce a reversal in responses. Note that the switch from a Type-1 to a Type-2 dominated response obtained in Fig. 5(a) and (b) is not possible in the cases of smaller Type-1 responses (not shown) for example when the coecients Kare taken to be smaller. For in those cases, small amounts of Type-1 cytokine have less eect on enhancing macrophage presentation. Thus, in the cases discussed in the previous section, too much of a given cytokine can cause downregulation of the response dominated by it.
4.3. Compartmentalization of immune responses
According to several studies, it seems that the spatial location of the immune response is a very important factor in the resulting cytokine balance. Findings of specialized environments in which dierent spectra of cytokine production occur bring Mossman (®nal discussion in [28]) to suggest a vaccine strategy in which two dierent immune responses might be induced in the same person by vaccinating simultaneously in dierent locations. In [72] it is argued that the physical com-partmentalization of the immune system may underlie the phenomenon of immunotherapy in allergy.
In a study aimed to evaluate Th-like cytokine patterns in dierent compartments of the body in patients with sarcoidosis, it was found that in the lungs, the cells shifted to the Th1 side of the spectrum, whereas cells in the peripheral blood exhibited intermediate cytokine pro®les [33]. Observations made in [73] point to the conclusion that the production of a speci®c cytokine pro®le is largely compartmentalized to distinct lymphoid organs. They demonstrate that in mice infected by T. spiralis, IFN-c producing cells predominate in the spleen, whereas IL-5 producing cells prevail in the mesenteric lymph nodes. (See also [74].) Similarly, a substantial body of experi-mental evidence indicates that routes of vaccination (oral, subcutaneous, etc.) can somehow de-termine the type of dominance (Th1 or Th2) of a resulting response. Inleishmania infections, for instance, a Th1 pattern was noted in CB6F1 mice infected in the footpad while a Th2 pattern developed in mice infected in the dorsal skin [32].
A variety of mechanisms have been oered to explain why cytokine responses may be com-partmentalized. According to one view, subsets of Th cells may display distinct homing charac-teristics via the expression of disparate homing receptors or adhesion molecules [75]. Another type of explanation conceives disparate populations of presenting cells that preferentially activate dierent subpopulations of T cells. This explanation relies on observations according to which Th1 and Th2 cells seem to dier in their requirement for the costimulatory signals provided by antigen presenting cells (reviewed in [23]).
Signi®cantly, studies seem to indicate that in dierent body locations, macrophages share dierent sensitivities to cytokines. For example, alveolar macrophages are activated by lower doses of
IFN-cthan peritoneal macrophages [76]. According to another study as little as 5 U/ml IFN-cinduces 50% intracellular killing activity in in¯ammatory macrophages, whereas 20 U/ml IFN-c induces an equivalent amount of intracellular killing in dierentiated tissue cells [77]. These experiments
Fig. 6. Eect of macrophagesÕcharacteristics on the systemÕs behavior. (a) Eect of the sensitivitiesK1;KM
1 (Eq. (7)) of macrophagesÕ cytokine secretion to Th and macrophage-derived Type-1 cytokines. (i) Small sensitivitiesK1 andK1M result in a Type-2 response. (ii) Mild sensitivities give a Type-1 response. (iii) Larger sensitivities result in a stronger Type-1 response. (b) Eect of the macrophagesÕpopulation size. (i) Small numbers,JM
1 ;J M 2 ;K
M 1 ;K
M
2 result in a Type-2 dominated response. (ii) Mild number give a Type-1 response. (iii) Larger numbers give a stronger Type-1 dominated response. In (a), b1b21; Ag1; ap's0:05; J's0:2; K2K20:2. In (ai) K1K1M0:06. In (aii) K1
KM
1 0:2. In (aiii)K1K1M0:6. In (b)J'sK's0:6; C1T 0 C2T 0 0:5. In (bi),J1MJ2MK1MK2M0:3. In (bii)JM
1 J M 2 K
M 1 K
M
2 0:6. In (biii)J M 1 J
M 2 K
M 1 K
are supportive of the view that the macrophagesÕsensitivity to cytokines might be a cause of the variability in outcome in dierent body locations. Fig. 6(b) shows that the macrophagesÕ popu-lation size can determine the type of the response. Certain conditions leading to a Type-2 dom-inated state in the case of small macrophage numbers can result in a Type-1 domdom-inated response in the case of large macrophage amounts, and in a stronger Type-1 response for even larger amounts. If we take under consideration the fact that the number of macrophage colonies dier in bone marrow, spleen, blood, lung (Table 2 in [78]) we have found another possible explanation for the variability in outcomes throughout the body.
Arti®cial intervention directed to the macrophage population can have important eects on the outcome of the immune response. For example, macrophages from diabetes-resistant donors were shown to prevent insulitis and diabetes in most non-obese diabetic mice recipients, `however the mechanism for protection is unclear', it is reported in [79]). I propose that the in¯uence of the set of macrophage properties stated above can be experimentally tested, and may provide a thera-peutic strategy in cases where an arti®cial intervention is needed in order to induce a desirable cytokine pattern type. Macrophage numbers might be arti®cially varied to render protection. MacrophagesÕsensitivity to cytokines can presumably be experimentally changed, for example by blocking suitable receptors.
Pathogens are known to be able to develop interesting and diverse strategies for survival in macrophages. In particular, it might be the case that pathogens have adopted such strategies as tampering with the sensitivity of the macrophages secretion and presentation abilities to dierent cytokines. For example,M. leprae's constituent LAM (lipoarabinomannan), produced in copious amounts in infected macrophages, apparently downregulates IFN-c-induced MHC-II expression and killing functions. (A similar eect is shared by L donovani, [80].) Moreover, defective re-sponses to IFN-c that were observed when macrophages harbored leprosy bacilli for 3±5 days were not observed in recently infected macrophages [81].
5. Summary
In this study, I argue that previous mathematical models attribute a secondary role to mac-rophages in the determination of the immune response. An attempt is made to take a further step in the direction of modeling feedback mechanisms between T helper cells and macrophages. Motivated by accumulating data on the complex interplay between these populations, I considered (i) the eect of macrophages on the T helpersÕcytokine secretion, exerted via non-speci®c cytokine communication, and via antigen presentation; (ii) the eect of T helpers on the macrophagesÕ abilities to secrete cytokines, and to present antigens. The model focused on how the feedback between both populations can determine the balance between a Type-1 and a Type-2 dominated response, allowing us to put forward new explanations to interesting phenomena (such as reversal, asymmetry, and compartmentalization of responses), which can be experimentally tested.
from the prevailing to the opposite type of domination. In the context of this ®nding, several points are worth stressing.
(i) Reversals in the dominance of a speci®c cytokine pro®le are indeed observed experimentally. In several diseases, this process is thought to be bene®cial to the host, so that mechanisms of auto-regulation may be at work. The model provides a possible feedback scenario to account for this auto-regulation process.
(ii) Feedback mechanisms that can induce a switch from an, e.g., Type-1 to a Type-2 dominated response, require a suciently strong Type-1 dominated response. Otherwise a Type-2 dominated response will not be attained. This seemingly paradoxical behavior stems from the fact that under certain conditions a reversal in responses can be induced by antigen-presentation mechanisms (mechanisms that are enhanced due to the prevailing cytokineÕs eect on the macrophages). However, (Section 4.2.2), this switch requires a large amount of prevailing cytokine: smaller amounts have less eect on enhancing macrophage presentation. Thus, too much of a given cy-tokine can cause downregulation of the response dominated by it. This rationale can account for paradoxical ®ndings indicating that Type-1 cytokines (such as IFN-gamma and NO) are re-sponsible for the downregulation of a Type-1 dominated response.
(iii) A particular aspect of the question reversibility, namely, asymmetry, was addressed. Ex-perimental data indicate that the Type-2 pro®le is more stable than its counterpart. As discussed in Section 4.2.1, one possible way to explain this phenomenon relies on dierent properties characterizing Th1 and Th2 subsets, for example dierent responsiveness to an extrinsic antigenic stimulus. However, the model proposed an additional scheme that relies on the sensitivity of macrophages to the in¯uence of Th cells. It was found that ifap1>ap2, i.e., if the sensitivity of the
macrophagesÕpresentation ability to Type1-Th-derived cytokines is larger than the sensitivity to Type-2-derived cytokines, then a switch from a Type-1 to a Type-2 dominated response is more likely to occur than the switch in the opposite direction. This type of asymmetry in the sensitivities is biologically plausible. Type-1 cytokines have on the average a positive eect on the macro-phagesÕpresentation ability, through their eect of MHC class II upregulation. With respect to Type-2 cytokines, it is believed that some cytokines comprising this category have positive eects on MHC class II expression, whereas others downregulate the expression of this molecule (see Table 1). Thus, averaging these properties may result inap1 >ap2.
An interesting point worth considering is the possible eect of the time evolution of the pa-rameters. How the system might automatically change its coecients in the context of choosing an appropriate immune response. As an example of experience-induced parameter modulation we note that as analyzed in [64], a large array of sensitivities to type-1 cytokines,
J1J1MK1K1M>J2J2MK2K2M tend to push towards a Type-1 dominated response.
Another observation that provided motivation relies on variability of responses according to dierent body compartments. We found (Section 4.3) that, for example, small or large values of sensitivities of macrophagesÕ functions (secretatory ± K or presentatory ± [ap]) to Th-derived cytokines can result in a Type-2 or a Type-1 dominated response, respectively. We showed that the macrophage population size can aect the type of response.
It is important to stress that the model represents a crude simpli®cation of the real feedback mechanisms that connect T helpers and macrophages. The diversity of cytokine production patterns has been reduced to a few main classes. The simplicity of the model makes its predictions necessarily qualitative in nature. In spite of its limitations, the model nonetheless seems to display explicative and predictive value. Examples of ®ndings that can be tested experimentally are the eect of the sensitivityap1(which can promote reinforcement or ± under other sets of conditions ±
reversal of the prevailing response), and the eect of the macrophage population size. It might well be the case that pathogens indeed tamper with the sensitivity of antigen presentation to cytokines, [ap], in order to subvert the immune response. Directed by physicians, such `tampering' may lead to the amelioration of disease.
Interestingly, the very recent ®ndings by Rissoan and colleagues [83,84] support the view presented in this paper concerning feedback loops between antigen presenting cells and T helper cells. It is suggested in their work that the responsibility for the crucial decision between Th1 or Th2 responses can be shifted from the T cells to the antigen-presenting cells. On the other hand, their results also indicate that T helper cells themselves may regulate Th1 and Th2 responses by determining the survival of the appropriate dendritic cell subset. Here, when modeling antigen presenting cells, we have limited ourselves to the consideration of macrophages (and not dendritic cells) due to the more extensive experimental data found on the former (see Table 1). Excitingly, the ®ndings of Rissoan and coworkers, together with future ®ndings, will provide more experi-mental data on dendritic cells, allowing for their incorporation in future models.
The picture stemming from my eort to incorporate feedback mechanisms between macro-phages and Th is that T helper cells can determine their own fate by means of intrinsic properties rather than extrinsic factors such as ÔtyrantÕ macrophages that preferentially activate e.g., Th1 than Th2 cells. Macrophages are noÔslavesÕeither, duly told by the T helpers whether and when to be cytotoxic. In an interplay of positive and negative signals, macrophages can similarly partic-ipate in determining their own fate by means of in¯uencing (via cytokines and via antigen pre-sentation) the in¯uences that aect them.
Acknowledgements
I am most grateful to Lee Segel for extremely stimulating discussions, valuable suggestions, and for his supervision during the writing of this manuscript. This research is supported by the United States±Israel Binational Science Foundation grant No. 95-00526.
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