An appraisal of T cell subsets and the potential for autoimmune injury

Abstract

The initiation of T cell dependent immune responses requires the T cell to utilize a heterodimeric cell surface antigen receptor to recognize an antigenic peptide in association with major histocompatibility complex (MHC) molecules on the surface of antigen presenting cells (APCs). This is a necessary but insufficient signal for T cell activation. T cells must additionally receive a costimulatory signal that can be delivered through a variety of receptor-ligand pairs. Of these, the best characterized is the CD28-B7 (CD80 and CD86) couple. It has been appreciated for decades that the outcome of the T cell-APC interaction is highly variable and can lead to different forms of immune responses. The past decade has witnessed major advances in the definition of how distinct lymphocyte functions are dictated by expression of distinct cytokines, activation of defined signal transduction pathways, and, most recently, expression of distinct transcription factors. The production of ever increasing numbers of induced mutant mouse strains has created new reagents in which to analyze the role of particular cytokines or other proteins in defined immune responses. While the temptation remains to accomodate emerging information into reductionist models of T cell dependent immunopathology, it is our view that in vivo immune responses are highly complex and regulated events that defy simple categorization.