Th cell-deficient mice control influenza virus infection more effectively than Th- and B cell-deficient mice: evidence for a Th-independent contribution by B cells to virus clearance.

The notion that MHC class I- restricted CD8+ T (Tc) cells are capable of resolving autonomously infections with influenza virus is based largely on studies testing virus strains of low pathogenicity in CD4+ T (Th) cell-deficient/depleted mice. To test whether this holds also for pathogenic strains and to exclude possible contributions by B cells, we analyzed PR8 infection in Th cell-depleted B cell-deficient (muMT) mice. These mice, termed muMT (-CD4), showed 80% mortality after infection with a small dose of PR8, which resulted in insignificant mortality in intact or Th cell-depleted BALB/c mice. Infection of muMT(-CD4) mice with a virus of low pathogenicity was resolved without mortality, but, compared with intact BALB/c mice, with delay of approximately 5 and approximately 20 days from lung and nose, respectively. The low mortality of Th cell-depleted BALB/c mice suggested that B cells contributed to recovery in a Th-independent manner. This was verified by showing that transfer of 8-10 million T cell-depleted naive spleen cells into muMT(-CD4) mice 1 day before infection reduced mortality to 0%. The mechanism by which B cells improved recovery was investigated. We found no evidence that they operated by improving the lung-associated Tc response. Treatment of infected muMT(-CD4) mice with normal mouse serum spiked with hemagglutinin-specific IgM did not reduce mortality. Taken together, the data show that 1) the Tc response is capable of resolving autonomously (in conjunction with innate defenses) influenza virus infections, although with substantial delay compared with intact mice, and 2) B cells can contribute to recovery by a Th-independent mechanism.