Studying immunological tolerance by physically monitoring antigen-specific T cells in vivo

Generation of antigen-specific immunity or tolerance are different outcomes of a highly complex interaction between antigen, antigen-specific lymphocytes, and APC. Clearly, full understanding of this process must include the study of antigen-specific lymphocytes in vivo, under conditions known to result in immunity or tolerance. This has now become possible with the advent of methods that allow direct detection of antigen-specific T cells. One of the most striking observations made thus far is the seemingly critical crosstalk between T and B cells. It is well established that B-cell responses are to a high degree dependent on T-cell help, which consists of CD40/CD40 ligand interaction and delivery of various T cell-derived cytokines. Several lines of evidence now point to equal dependency of T-cell responses on interaction with B cells. T helper-cell priming in lymph nodes has only been observed when these cells migrate into B cell-rich follicles. In addition, T-cell priming does not occur in anti-IgM-treated B cell-depleted mice, and adoptive transfer of B cells back into these animals before immunization results in restoration of T-cell responses. Finally, when antigen presentation is targeted to occur exclusively through B cells, the effect on T cells seems to depend on the activation state of B cells. T-cell immunity results when B cells are activated, whereas tolerance is induced when the B cells are resting. The following model outlines possible events that can lead to tolerance or immunity, with an emphasis on the role of B-cell APC. Tolerance is induced when monomeric soluble antigens are injected without adjuvants. Antigen is taken up by dendritic cells that constitutively express costimulatory molecules (e.g., B7) and are thus able to stimulate IL-2 production and some T-cell proliferation. T-cell proliferation is short lived, and T cells are restricted to the paracortex, where they subsequently encounter resting B cells that present antigen leading to tolerance. In the case of oral tolerance, it is also possible that the GALT is the major source of B cells that carry antigen throughout the peripheral lymphoid tissues. Because antigens picked up in the GALT are predigested in the gut lumen, activation of B cells by cross-linking their surface immunoglobulin receptors may be particularly unlikely. Adjuvants, however, are able to shift the sequence toward immunity by activating B cells to express costimulatory molecules. Adsorption of antigens to alum may do the same thing by enhancing cross-linking of B cell-surface immunoglobulins. It is also conceivable that signals generated by the antigen and adjuvant may directly act on T cells and promote migration to the follicles, where interaction with activated B cells would be more likely to take place. This would lead to further activation and proliferation of T cells (that subsequently would leave the lymph node and migrate toward the tissues) and stimulation of antibody production by the B cells. This model places great emphasis on the role of adjuvants in the induction of immunity to soluble antigens. How then is immunity ever induced to infectious agents that obviously do not enter the body emulsified in CFA? The answer probably is that molecules with adjuvant properties (such as lipopolysaccharide, peptidoglycan, or double-stranded RNA) are intrinsic components of all microbes that the innate immune system has come to recognize. If this model is correct, then peripheral tolerance is actually the default pathway that the immune system will follow unless the antigen in question is recognized in an inflammatory context. The advantage of this strategy is that any newly expressed self-protein will induce tolerance, and only antigens that are recognized in the context of inflammation will induce immunity.