response to increasing doses of IL-2, but when restimulated by increasing doses of antigen there is a substantial decline in the number of viable cells. The exception is at low doses of IL-2, and even here proliferation drops off in response to higher doses of antigen, which induce endogenous IL-2 that supplements the existing level in the culture.
This finding—that the things that stimulate T-cells (i.e., antigen receptor occupancy and IL-2) can also program them to die—creates a paradox. After infection with a replicating pathogen, one might hope that T-cell response would increase along with the pathogen challenge. Researchers now believe that this is negative feedback regulation: the system appears to recognize that a strong immune response represents a potentially dangerous alteration in normal physiology, and therefore modulates the response in a very specific way that is tied to the particular antigen and the amount of stimulation received. There are, in fact, many examples of noncytopathic (parasitic) viruses in which the immune response is the detrimental component of the disease, rather than the replication of the microorganism itself. It is also known from clinical trials that several of the T-cell-derived cytokines are very toxic in the high doses that would result from an unregulated response to heavy antigen load.
Researchers tested this hypothesis by repeatedly challenging BALB/c mice with a superantigen, staphylococcal enterotoxin B (SEB), that is similar to toxic shock toxin. They administered large amounts of SEB to initiate immune response, followed by smaller doses on days 3 and 5 to maintain a large population of cycling cells, while also giving the animals regular doses of an antibody that would block the IL-2 receptor and thus prevent IL-2 responses. They sacrificed the animals at day 8, harvested the lymph nodes, and measured the populations of V-beta-8 T-cells (which specifically respond to SEB) and V-beta-6 T-cells (which do not). The results showed that the fraction of V-beta-8 cells dropped from 22 percent to 7.5 percent—a deletion of 60 to 70 percent of specifically responding T-cells through exposure to the very superantigen that caused them to undergo mitogenesis.
Evidence of Active Programmed T-Cell Death. This finding, which has been reproduced by other groups and with peptide antigens, fits a feedback regulation model that researchers call “propriocidal regulation,” a name borrowed from neural feedback regulatory loops. Activated T-cells both produce and respond to IL-2, and when cycling or proliferating T-cells are rechallenged by high doses of antigen a fraction of them will be shunted into an active programmed death pathway. This is not a mechanism to mop up T-cells at the end of an immune response, when the antigen is cleared—it occurs during the immune response and is a means of regulating that response when antigen is still present.
On the molecular level, active programmed death requires strong engagement of the T-cell receptor, which stimulates high-level production of death-inducing cytokines, including tumor necrosis factor (TNF) and Fas ligand. These cytokines engage specific receptors on the cell surface, engaging intracellular mechanisms that produce apoptotic proteins. Through several steps that have not yet been characterized, this leads to the activation of a cystine protease that is