Mice infected with Schistosoma mansoni develop a novel non-T-lymphocyte suppressor population which inhibits virus-specific CTL induction via a soluble factor

We previously observed that Schistosoma mansoni-infected mice were deficient in their ability to mount a CTL response to unrelated viral antigens and to clear a vaccinia viral infection. Here, we explore the mechanism of that deficiency. Mixing experiments showed that splenocytes from S. mansoni-infected mice actively suppress stimulation in vitro of both viral-peptide specific CTL in spleen cells from virus-infected mice, and allospecific CTL. The mechanism of suppression involves at least in part a soluble factor, in that it can occur across a 0.4-μm membrane which prohibits direct cell contact. However, the inhibition is not alleviated by blocking with antibodies to IL-4, IL-10 or TGF-β. Fractionation of the splenocyte population from S. mansoni-infected mice shows that the suppression is mediated by a non-B, non-T cell that expresses CD16 and Mac-1, but not FcϵR or NK1.1. This represents a novel suppressor population that is distinct from the FcϵRI + populations of non-B, non-T cells in the spleens of S. mansoni-infected mice that provide a major source of IL-4 in these animals. Similar cells in schistosome-infected humans could affect susceptibility to other infections or responsiveness to vaccines.