Do L3T4+ T cells act as effector cells in protection against influenza virus infection.

This study aimed to analyse the roles of Lyt 2+ and L3T4+ memory T-cell subpopulations in murine influenza infection. Previous work has shown that Lyt 2+ cytotoxic T-cell (Tc) clones can adoptively transfer protection. We therefore wished to see whether L3T4+ (Th) cells could also act as protective effector cells. Donors for adoptive cell transfer were thymectomized mice, depleted in vivo of either Lyt 2+ or L3T4+ T cells with monoclonal antibodies (MAb) and then infected with influenza virus (A/X31). Primed spleen cells, after removal of the B cells, were transferred into irradiated hosts infected simultaneously or persistently with a heterologous influenza virus and the effect on lung virus replication determined. Depletion of L3T4+ T cells suppressed the formation of IgG antibodies after influenza virus infection, indicating significant depletion of T-helper function. Yet Lyt 2+ class I MHC-restricted Tc cells were effectively primed in these mice, albeit to half the normal level. Adoptive transfer of the Lyt 2+ memory T cells cleared virus in a persistent infection within 6 days. Spleen cells selected for L3T4+ T cells cleared virus within 21 days of transfer in a simultaneous infection and reduced viral titres in a persistent infection, but not as effectively as L3T4+-depleted spleen cells. Although no Lyt 2+ cells were detected by fluorescence staining in Lyt 2+-depleted spleens, we could detect low levels of class I MHC-restricted influenza-specific Tc memory cells in host spleens following influenza infection. Therefore, whether the early viral clearance is solely due to L3T4+ T cells is not clear. Lyt 2+ memory T cells appear more efficient in this respect than L3T4+ memory T cells.