M Schaefer

Functions of Purified L3T4+ and Lyt-2+ Cells in vitro and in vivo

The expression of L3T4 and Lyt-2 cell surface molecules separates T cells into two broad, non-overlapping subsets: typical T helper cells are L3T4+ Lyt-2- whereas most T killer cells and their precursors are L3T4- Lyt-2+. This review compares highly purified populations of unprimed L3T4+ and Lyt-2+ cells for their capacity to respond to class I vs. class II H-2 alloantigens. Various parameters are considered, including generation of mixed lymphocyte reactions (MLR) and cell mediated lympholysis (CML) in vitro, proliferative responses in irradiated mice, graft-versus-host reactions and skin allograft rejection. In all of these assays the two T cell subsets exhibit marked specificity in their response to H-2 alloantigens, L3T4+ cells responding only to class II and not class I differences and Lyt-2+ cells showing reciprocal specificity. Contrary to current dogma, the bulk of the evidence suggests that primary responses of Lyt-2+ cells do not depend on exogenous help provided by other T cells.

Antigen-presenting cells for unprimed T cells

The triggering requirements of T cells differ for primed and unprimed cells: primed T cells can be triggered to produce lymphokines without viable antigen-presenting cells (APCs), apparently by crosslinking the T-cell receptor (TCR). Unprimed T cells do, however, require viable APCs and here Jonathan Sprent and Mary Schaefer review what type of cells can carry out this function, with particular reference to APCs for unprimed CD8+ cells.