Subsets of CD4+ T cells and their roles in autoimmunity

The CD4 molecule has a very restricted tissue distribution being found at high levels only on subpopulations of thymocytes and peripheral T cells. This finding implicated the molecule in the specialized actions of these cells and provided the impetus for studies directed at determining the function of the CD4 molecule itself and of those T cells that expressed it. The first part of this paper reviews briefly some of the earlier work in this field in which Alan Williams played such a major role. The paper concludes with an account of more recent findings which reveal that CD4+ T cells are themselves phenotypically heterogeneous and that the different subsets that can be identified mediate markedly different immunological functions. In particular studies with laboratory rats have shown that one subset plays an essential role in the prevention of autoimmunity. This finding indicates that self tolerance cannot be accounted for entirely in terms of the deletion or irreversible inactivation of autoreactive T cells and raises a number of questions about how the immune response to self antigens is actively regulated and how possible deficiencies in this regulation may give rise to autoimmune disease.