Rick Tarleton
9.4 Adaptive immunity
The adaptive immune mechanisms controllingT. cruziinfection are specific, highly potent and multi-functional. They are largely successful in controlling
parasite replication, preventing early death in the acute phase and maintaining a low parasite burden in the chronic infection. T. cruziinduces (and is con- trolled by) a combination of cell-mediated and humoral immune responses, dominated by Th1 cytokines, lytic and opsonic antibodies and CD8+ cytolytic T cells. The absence of any one of these mechanisms results in the inability to regulate parasite growth and subsequent death of the host.
9.4.1 T helper cell responses
T. cruziinduces a relatively polarised type 1 T helper cell response with signifi- cant production of IFN-␥and TNF-␣. Although low levels of type 2 cytokines are sometimes observed, blocking or inhibition of type 2 cytokine responses has essentially no effect on either control of parasite load or the intensity of inflam- matory reactions. In contrast, defects in the ability to generate Th1 responses (by blocking IL-12 production, the signalling molecules involved in the gen- eration of Th1 responses or IFN-␥production itself ) prevents mice from con- trolling the acute infection. In fact, preventing production of IFN-␥creates one of the strongest susceptibility phenotypes in mice infected byT. cruzi, nearly equivalent to the effect of the absence of T cell responses in general.
CD4+ T cells are likely participants in the control of T. cruzi infection via a number of mechanisms, including IFN-␥enhanced macrophage-mediated try- panocidal activity, promotion of antibody production (see humoral immune responses, below), and the potentiation of inflammatory responses. Notably, CD4+ T cells are not required for the generation of functionalT. cruzi-specific CD8+ T cells, but CD8+ T cells that are not ‘helped’ by CD4+ T cells are present in lower frequency and fail to control the acute phase infection.
9.4.2 CD8+ T cell responses
T. cruziinvades and replicates in a number of host cell types that lack the in- ducible microbicidal functions of phagocytic cells. The ability to invade muscle (and other tissues) has sometimes been considered an immune escape mech- anism. However, it is clear thatT. cruzi-infected host cells do not completely evade immune detection, and that T cells that are responsible for the recogni- tion and destruction of pathogen-infected host cells – the CD8+ T cells – are key to the immune control ofT. cruziinfection.
Within its cytoplasmic niche in host cells,T. cruzireleases proteins that are pro- cessed and presented in association with host cell surface MHC-I, the targets for recognition byT. cruzi-specific CD8+ T cells. CD8+ T cells responding to theseT. cruziepitopes exhibit cytolytic activity, produce IFN-␥(Figure 9.2), can transfer a degree of protection in mice and dominate the inflammatory sites in acute and chronically infected hosts. The presence of CD8+ T cells at these sites of inflammation has also been argued to indicate their role as drivers in the development of disease pathology. However, the finding that depletion or blockage of the functions of these cells results inincreasedparasite load and disease severity argues for a disease-protective role for CD8+ T cells, rather than a disease-promoting one.
CD8+Tc CD8+Tc
CD8+Tc
Perforin Granzymes
IFN-γ
TCR
MHC-I / T. cruzi-derived peptide complex
Figure 9.2 CD8+ T cells regulate the growth ofT. cruziin infected cells, performing a crucial role in the control of infection.The relative sizes of the CD8+ T cells and the muscle cells have been equalised for illustrative purposes but, in reality, the muscle cells are much larger than CD8+
T cells. The mechanism by which CD8+ T cells regulate parasite growth is not fully understood, but it is assumed partly to occur via lysis of the infected muscle cells.
Abbreviations: IFN, interferon; MHC, Major histocompatibility complex; Tc, Cytotoxic T cell;
TCR, T cell receptor.
The natural target of a significant proportion of theseT. cruzi-specific CD8+ T cells are GPI-anchored surface proteins and, in particular, trans-sialidase (ts) proteins. In certain parasite-host strain combinations, more than 30 per cent of the total CD8+ T population at the peak of the infection is specific for a few ts peptides. This degree of immunodominance is remarkable, especially consid- ering that theT. cruzigenome is large and complex, with over 12,000 haploid genes, thousands of which are in the ts family.
The high frequency of ts-specific CD8+ T cells in T. cruzi-infected mice has made it possible to monitor the generation and persistence of this parasite- specific response closely during infection and under various conditions.
Throughout the long course of infection in mice, theseT. cruzi-specific T cells maintain a mostly short-lived effector/effector memory phenotype, indicative of the presence of an active infection with at least occasional antigen encounter.
Upon cure of the infection using the anti-T. cruzidrugs, the parasite-specific CD8+ T cells assume a predominantly long-lived/central memory phenotype.
Unlike many other chronic infections, theT. cruzi-specific CD8+ T cell response in mice does not show signs of immune exhaustion, even after more than two years of infection. This result reflects again the low antigen load in the chronic
stage of the infection, and provides additional corroboration that the immune system is successful in the long-term control of the infection.
Although the size and effectiveness of the anti-T. cruziCD8+ T cell response attests to its high effectiveness, the initial generation of the response appears to be significantly delayed, relative to that seen in many other infections. This observation is consistent with the evidence, discussed above, that the infec- tion byT. cruzi is relatively silent due to the absence of strong triggering of innate responses.
9.4.3 Humoral immune responses
Antibody responses during T. cruzi infection have often been characterised as non-specific (polyclonal), but this is not uniformly the case. It may simply reflect the vast number of antigen variants that T. cruzi presents to the im- mune system through an array of multigene families of surface proteins such as the ts, mucins, and mucin-associated proteins (MASPS). Transfer of serum, or antibody fractions of serum from infected animals, provides significant (al- though not total) protection to infection in na¨ıve animals. Furthermore, ani- mals lacking B cells are highly susceptible to infection although, interestingly, mice lacking B cells due to a knockout in the mu immunoglobulin heavy chain (MT) are able to control acute infections somewhat longer than do mice lacking CD8+ T cells before eventually succumbing to the infection with very high parasitaemias.
The mechanism of antibody-mediated control ofT. cruziinfectionin vivois not fully understood, but anti-T. cruziantibodies have been demonstrated to par- tially block host cell invasionin vitro, to facilitateT. cruziuptake by phagocytic cells and to induce complement-mediated and complement-independent lysis ofT. cruzi(Figure 9.3).
Two rather unique reported activities in the pool of lytic antibodies induced byT. cruziinfection are anti-galactosyl antibodies that lyseT. cruziwithout the participation of the classical or alternative complement pathway, and antibod- ies against a complement regulatory protein (CRP) that act by preventing the normal decay-accelerating factor-like activity of this CRP (see Chapter 1), in turn facilitating complement lysis ofT. cruzi parasites. These lytic antibodies have been suggested as good markers of spontaneous or drug-induced para- sitological cure inT. cruziinfection