BALB/c mice infected with Brucella abortus express protracted polyclonal responses of both IgG2a and IgG3 isotypes.

A polyclonal IgG2a response dependent on the secretion of endogenous IFN-gamma has been demonstrated in BALB/c mice injected with killed whole cells of Brucella abortus [1]. Here we report intense and protracted polyclonal responses of IgG2a and also of IgG3 isotypes in BALB/c mice undergoing primary infections with B. abortus attenuated vaccine strain 19 or virulent strain 2308. Ratios of total serum Ig levels between infected mice and age matched controls were greater than 38 for IgG3 and greater than 12 for IgG2a between weeks 4 and 8 post-infection. Polyclonal increases of IgM and IgG1 that were proportionally much lower (ratios < 2 and < or = 3, respectively) also occurred in infected mice during this time. It is hypothesized that both IgG3 and IgG2a polyclonal responses required IFN-gamma, which was induced by B. abortus primarily in a T cell-independent fashion during the first weeks of infection, and from T cells thereafter.