T helper cells in cytotoxic T lymphocyte development: analysis of the cellular basis for deficient T helper cell function in the L3T4-independent T helper cell pathway.

In this present study, lymphokine (IL-2/IL-4) production in VSV-induced Th cell (L3T4+Lyt-2- VSV-immune T cells) and memory CTL populations (L3T4-Lyt-2+ VSV-immune T cells) has been assessed in order to gain some understanding as to why the Lyt-2+ subset (L3T4-independent Th cell pathway) fails to provide Th cell function for anti-VSV CTL responses. Our studies demonstrated that following specific antigen (VSV, H-2 antigen) or mitogen stimulation, lymphokine activity was detected in the supernatants obtained from VSV-induced Th but not VSV memory CTL populations. The presence of blocking concentrations of PC61, a monoclonal antibody (mAb) to the IL-2 receptor (IL-2R), revealed augmented lymphokine activity only in the VSV-induced Th cell supernatant. VSV-induced Th cells secreted both IL-2 and IL-4 following stimulation with VSV. Two lines of evidence supported the view that both these lymphokines were important for an anti-VSV CTL response: (1) mAb to either IL-2 or IL-4 inhibited CTL maturation and (2) the combination of exogenous IL-2 and IL-4 reconstituted a class I-restricted. VSV-specific CTL response in Th cell-depleted T cell cultures. The failure to detect lymphokine production in bulk cultures of the VSV memory CTL population was consistent with limiting dilution (LD) analysis of lymphokine-producing cells in the spleen of VSV-immune mice. Thus, approximately 1/15,000 Lyt-2-depleted, VSV-immune T cells were positive for lymphokine production following VSV stimulation, whereas less than 1/1,000,000 L3T4-depleted, VSV-immune T cells were scored as lymphokine-secreting cells following stimulation with this same virus. Similarly, precursor estimates for lymphokine-producing cells against allogeneic class I antigens demonstrated that the majority of lymphokine-producing cells also resided in the L3T4+ subset. Lymphokine-secreting Lyt-2+ cells were detected at low but not high cell densities suggesting that Lyt-2+ cells may secrete another lymphokine(s) that inhibits IL-2/IL-4 production. Thus, these studies demonstrate an obligatory requirement for the L3T4-dependent Th cell pathway for optimal CTL responses derived from either CTLp or memory CTLs.