The DTH-initiating Thy-1+ cell is double-negative (CD4-, CD8-) and CD3-, and expresses IL-3 receptors, but no IL-2 receptors.

The elicitation of delayed-type hypersensitivity (DTH) requires an early-acting Thy-1+ cell that produces an Ag-specific, non-MHC-restricted factor that initiates DTH by sensitizing the local tissue for release of the vasoactive amine serotonin. We characterized the phenotype of this DTH-initiating cell by treating cells from sensitized mice with different antibodies and then either with rabbit C or anti-Ig panning or bead separation to deplete various subpopulations. We then transferred these cells i.v. into naive recipients that were challenged to elicit DTH. Our findings indicate that the early DTH-initiating cell is Thy-1+, Lyt-1+, CD4-, CD8- and CD3-, whereas the classical, late DTH effector T cell is Thy-1+, Lyt-1+, CD4+, CD8-, and CD3+. We hypothesize that DTH-initiating cells are primitive T cells with Ag receptors that can bind Ag without MHC-restriction. This hypothesis was supported by the finding that two different antibodies, that both bind T cell-derived Ag-binding molecules, eliminated the DTH-initiating, cell but did not affect the late component, MHC-restricted CD4+, CD3+ T cell. Additional experiments with antibodies against restricted determinants of the T-200 glycoprotein family (CD45R) showed that the early but not the late cell is positive for B220, which is usually present on B cells, and on some activated T cells. Also, the DTH-initiating cell is Il-2R-, but Il-3R+; whereas the late component DTH T cell is IL-2R+ and IL-3-. Our findings suggest that DTH-initiating cells may be Ag-specific lymphoid precursor cells that arise before final differentiation along the pathway leading to mature T or B cells. Our results indicate that antigen-specific Thy-1+, CD3-, CD4-, CD8- cells function in vivo to initiate DTH reactions.