Limiting dilution cultures reveal latent influenza virus-specific helper T cells in virus-primed mice.

Although lymphokine producing helper T cells are suspected to play an important role in the development of anti-viral cytotoxicity, they have not previously been demonstrable in influenza virus-primed mice unless the mice have first been pretreated with cyclophosphamide. It has been assumed that cyclophosphamide pretreatment was required to block the activities of suppressor cells which would otherwise interfere with helper T cell priming. We have developed a limiting dilution assay (LDA) for estimating influenza virus-specific T cell precursors for IL-2 secretion ("pHTL"), and have found that equal numbers of pHTL develop in virus-primed mice regardless of cyclophosphamide pretreatment. This result suggests that the cyclophosphamide-sensitive regulatory cells do not act in vivo to prevent helper cell priming, but rather act in vitro to oppose expression of helper cell activity. Comparison of helper cell function in conventional (high density) cultures and in LDA cultures reveals an Lyt-2+ cell which prevents development of cytotoxic function in conventional assays, but which fails to affect the outcome of the LDA experiments. We found that the pHTL population consists almost entirely of Lyt-2- cells, and that mice primed with the Type A strain, PR/8, develop pHTL responsive to a wide range of influenza viruses, including a Type B strain. Both results modify conclusions suggested by conventional assay methods. Since the LDA experiments seem to be relatively resistant to regulatory interactions which can influence the outcome of standard assay cultures, they can supply information about the characteristics of the pHTL itself that would otherwise be difficult to obtain.