B cell-deficient mice are highly resistant to Leishmania donovani infection, but develop neutrophil-mediated tissue pathology

Resolution of Leishmania infection is T cell-dependent, and B lymphocytes h ave been considered to play a minimal role in host defense. In this study, the contribution of B lymphocytes to the response against Leishmania donova ni was investigated using genetically modified IgM transmembrane domain (mu MT) mutant mice, which lack mature B lymphocytes, When compared with wild- type mice, mu MT mice cleared parasites more rapidly from the liver, and in fection failed to establish in the spleen. The rapid clearance of parasites in mu MT mice was associated with accelerated and more extensive hepatic g ranuloma formation compared with wild-type mice. However, the liver of infe cted mu MT mice also showed signs of destructive pathology, associated with the presence of increased numbers of neutrophils. The role of neutrophils in controlling parasite growth in the viscera was determined by depletion w ith the mAb RB6-8C5, This treatment led to a dramatic enhancement of parasi te growth in both the liver and spleen of mu MT and wild-type mice. As asse ssed by transfer of both normal and chronic-infection serum, Ig protects mu MT mice from destructive hepatic pathology, but minimally alters their res istance compared with wild-type mice, However, adoptive transfer of CD4(+) and CD8(+) T cells into recombinase activating gene 1 (RAG1(-/-)) recipient s, suggested that T cell function was not altered by maturation in a B cell -deficient environment. Taken together, these data suggest an inhibitory ro le for B lymphocytes in resistance to L, donovani unrelated to the presence or absence of Ig. However, Ig protects mu MT mice from the exaggerated pat hology that occurs during infection.