T-cells thus occur in several types which have effector and /or regulatory functions. T helper lymphocytes (CD3+, CD4+ cells) are central to the immune system and can be thought of as the “conductors” of the immune orchestra; any microbe that impedes or destroys CD3+, CD4+ T lymphocytes will cause the immune system to lose order and eventually degenerate. Helper T-cells are key “organisers” of both humoral and cell- mediated adaptive immunity, acting mainly through release of a range of cytokines, and inter alia enabling cytotoxic T-cells to do their work, B-cells to differentiate and develop
into plasma cells, and macrophages to destroy pathogens they have ingested through phagocytosis. Cytotoxic T Lymphocytes (CTL; CD3+ CD8+ cells) specifically kill target cells identified in a complex way (see below) as containing an organism bearing a specific antigen. They do this by perforating the target cell membranes by secretion of perforin and cytolysins, and by releasing other cytotoxic substances directly into the dying target cells. In healthy humans, CD3+CD8+ lymphocytes make up the smaller proportion of CD3+ T-cells, but a homeostatic balance exists between CD4+ and CD8+ T-cells, in that the size of the total pool of T-cells (as marked by CD3) is apparently maintained by a variety of mechanisms. As mentioned above, Regulatory T-cells (Tregs, characterised by the surface marker CD25 and previously known as suppressor T-cells) usually but not always limit the extent and may change the nature of certain immune responses, partly through cytokine release, partly through competitive effects and partly through direct cell-cell interactions12.
The ability of the adaptive immune system to respond to invasion of the body by a pathogen depends critically on events that link the antigens exposed on the surface of the pathogen concerned with the production within a few days of both soluble antibodies (produced by vastly expanded clones of plasma cells derived from activated B-cells) and cytotoxic T-cells (generated likewise by clonal expansion of a subset of T-cells) directed specifically at the foreign antigen set. Some of these events are part of the innate immune response already described, especially those involving the phagocytic action of macrophages; others are associated with the special capabilities of “sentinel cells” called dendritic cells located strategically in parts of the body where infections are likely to occur, in mucous membranes of the airways, the gut, genitourinary tract, etc. Both macrophages and dendritic cells are “antigen-presenting cells” (APC) in that they digest complex pathogen-derived macromolecules to fragments, some of which will be the actual antigens or, more correctly, epitopes, to which small subsets of the vast numbers of B and T lymphocytes have specificity. (Sometimes, the presenting cells are coated with the foreign antigenic pathogen, as in the case of HIV – see below). When pathogens have multiplied outside body cells and been ingested by phagocytosis, as previously described, their digestion in the phagolysosomes is the route of antigenic fragment generation, and the processed fragments are bound tightly and stably to the specific peptide-binding cleft found in proteins of the major histocompatibility complex (MHC) class II, and lodged in the cell membrane to act as the “presented antigen”. When pathogens have gained access to target cells and are replicating inside them, it is the intracellular processing or turnover of their antigenic molecules by the infected cells that generates the epitope-bearing processed fragments which become bound in this case to MHC class I molecules, which after membrane localisation present the specific foreign antigens to the external environment as a tell-tale signal of what is lurking within.
The peptide clefts on MHC proteins of both classes available for binding antigenic fragments, while each possessed of binding rules, are promiscuous enough to make it likely that at least one or more antigens will in fact be presented on the cell surfaces of APC’s after any pathogen infection. The limits to this, and the patterns of responsivity that are imposed by the particular MHC gene alleles present in an individual, are called restriction, and are extremely important determinants of host susceptibility to particular pathogens (a good example is differential susceptibility to HIV infection, and of disease progress, associated with possession of certain alleles of the HLA-B genes in the MHC class I group)14. Only T-cells through their unique T-cell receptors (TCR) can recognise the antigens presented by APCs: cytotoxic T-cells (marked as such by containing the surface co-receptor membrane protein CD8) recognise MHC class I-bound antigenic peptide fragments, while T-helper cells (bearing the co-receptor marker CD4) recognise antigens bound to MHC class II molecules on presenting cells.
Activation of T-cells (the inductive phase of adaptive immunity) is a complex, highly regulated set of processes, to prevent accidental assaults on the body’s own tissues (normally completely abrogated by developmental mechanisms not covered here) and to lessen the likelihood that pathogens co-evolving with human populations will find ways of subverting or even eliminating the arsenal of complementary attacking possibilities made possible by the highly evolved human adaptive immune system. The activation or inductive site is lymphoid tissue, whether this be in the intestines (gut-associated lymphoid tissue, GALT), regional lymph nodes or the spleen. Immature dendritic cells are the “professionals” in antigen presentation as described above, continuously sampling with great sensitivity both soluble microbial products (through micropinocytosis) and the whole microbes themselves, recognising the pathogens through Toll-like receptors or because they have been opsonised (see above), and internalising them through phagocytosis for antigen processing and presentation through (MHC class II) or after becoming infected by internally replicating organisms (MHC class I). In either case, they become motile and migratory in their behaviour, so that they very rapidly end up in lymphoid tissue, mainly via lymph drainage pathways. Certain pathogens (e.g. HIV) can also be externally bound to dendritic cells in MHC class I-restricted ways that also cause migration and are stable enough to survive the journey to lymphoid tissue. Maturing dendritic cells secrete cytokines including chemokines, attracting to, and activating other cell types at, the site of infection (see above, innate immunity), while pre-emptively upregulating T-cell co-stimulatory molecules on their surfaces, such as CD4+.
Mature dendritic cells once in lymphoid tissue are potent activators of T-cells, each being able to activate 100–3000 T-cells. Productive interaction is initiated when T-cells bear the “correct” TCR encounter and bind to APCs bearing the matching antigenic peptide fragment signalling the presence in the body of a particular pathogen. The
number of co-receptors and adhesion-enhancing molecules provided by both interacting cells increases to intensify the close binding of the two cell types, until, via highly complex signalling pathways, the proliferation and lymphoblastic transformation of the affected T-cells into specific effector T-cells of the adaptive immune system is induced.
These cells may be of two types, dependent on complex differential cytokine stimulation patterns: Th 1, assisting mainly with cytotoxic reactions, and Th 2, focusing inter alia on B-cell differentiation. Effector CD8+ T-cells (cytotoxic T-cells) kill cells bearing MHC class I-bound, pathogen-derived antigenic fragments, as already described. Effector CD4+ T-cells (T helper cells) amongst other actions are able to activate macrophages bearing MHC class II-bound, pathogen-derived antigenic fragments, in the presence of interferon-gamma secreted by the engaged T-cells, and by co-stimulation by their CD4+. These activated macrophages have greatly enhanced anti-microbial capacity through enhancement of the phagolysosomal pathway, induction of “respiratory bursts”
(destructive oxygen radicals- see above), stimulation of nitric oxide formation, and increased formation of antimicrobial proteins such as defensins15. The fact that all this activity is directed by antigen-specific effector T-cells only at macrophages presenting pathogen-derived antigens keeps damage to host tissues potentially caused by secreted immune mediators to a minimum.(It is important, however, to realise that certain chronic infections, like those caused by pathogenic mycobacteria like those which cause tuberculosis and leprosy, can be associated with considerable tissue destruction by immune mechanisms which can become lethal if massive haemorrhages occur or vital structures are affected).
Some highly repetitive pathogen antigens (especially polysaccharide antigens) can stimulate resting B-cells into producing antibodies without the involvement of effector T-cells. Most B-cell responses, however, require T helper functions to enable the full expression of antibody-associated phenomena needed in humoral adaptive immune responses, including the ability to switch the kind of antibody molecule being produced (important for mucous membrane-lined cavities in the body where immunoglobulin A, IgA, is needed as a transmucosally acting secretory antibody capable of attacking pathogens at their site of entry16). Resting B-cells internalise such pathogen-derived antigen-immunoglobulin complexes as are formed in the initial, relatively ineffective stages of a humoral response, and peptide fragments presented on MHC class II molecules are then recognised by T-cells with specificity towards the same antigen; this, in the presence of co-receptors and cytokines, results in B-cell activation, differentiation into plasma cells, and production of high-affinity antibodies, all as already described. The T-cell stimulation also permits affinity maturation to take place, so that the antibodies formed at the height of the infection, and “remembered” in memory cells, are the most effective ones.
Antibodies are powerful agents of anti-pathogenic defence: they bind to and neutralise
soluble products (e.g. toxins) released by pathogens, to form (often) insoluble complexes that can be removed by phagocytes; they bind to target antigens on microbial surfaces, efficiently opsonising the particulate organisms for phagocytic attack; they potentiate the action of natural killer cells by identifying and tagging cells harbouring replicating infecting agents; and they abet complement attack (see above).
Activated lymphocytes are generally shortlived and undergo apoptosis as a naturally occurring mechanism limiting the immune responses concerned. Some survive, however, and continue life as CD62+ memory cells, able to mature into a proliferated population of specifically targeted effector cells as soon as the offending antigen/pathogen is again encountered in the body. In chronic infections such as HIV and tuberculosis, however, the effector CTLs fail to clear the body of the infecting organism and progressively dwindle in number and effectiveness – they become exhausted. A surface protein called PD-1 accumulates in these circumstances and responds to a circulating cytokine PD-1L to produce the “exhausted” phenotype. Fortunately, interrupting the PD-1D-1L interaction (with an antibody) can “revive” the lymphocytes17,as has been shown in HIV- infected subjects18.