The quantal theory of immunity and the interleukin-2-dependent negative feedback regulation of the immune response

The regulation of the tempo, magnitude, and duration of the immune response has been thought to reside solely with antigen for the past 50 years. However, with the discovery of the interleukins (ILs) 30 years ago, it became evident that these endogenous 'lymphocytotrophic hormones' provide the molecular mechanisms via classic hormone-receptor interactions. However, lacking in the hormonal regulatory capacity of the ILs were negative feedback mechanisms that functioned to switch off the positive driving force of the immune response, whether after antigen was cleared or when antigen persists, as with auto-antigens, tumor antigens, persistent infections, or allografts. Our recent experimental data, reviewed herein, exploring the T-cell antigen receptor (TCR) induction of the negative transcriptional regulator, forkhead box protein 3 (FOXP3), indicate that its expression is signaled by the T-cell growth factor IL-2. Once expressed, FOXP3 functions to restrict IL-2 expression in reaction to continued TCR stimulation. Thus, IL-2 regulates it own levels via a FOXP3-mediated negative feedback loop. In contrast, we found no evidence that FOXP3(+) cells actively suppress IL-2 expression, thereby failing to support the notion that such cells regulate potential effector cells.