Expression of FcμR by bovine mononuclear blood leukocytes.

The receptor for IgM has been identified a few years ago, but its expression by bovine mononuclear cells has not yet been studied. We used rabbit antibodies against bovine FcμR to begin to fill this gap. Anti-FcμR antibodies bound to B lymphocytes and monocytes, although less than to neutrophils. Nonclassical and intermediate monocytes (CD172a CD16) bound nonspecifically to rabbit antibodies, complicating analysis, but they bound more anti-FcμR antibodies than control antibodies, indicating that they also express the FcμR. They also express more C5a receptors than classical monocytes. Anti-FcμR antibodies did not bind to CD3 αβT lymphocytes (both CD4 and CD8) and γδT cells. At low temperature but not at physiological temperature, purified bovine IgM bound to all monocytes and strongly to all B cells, but hardly to CD3 T cells. Monocytes and B cells bound human IgA, but IgA did not compete, whereas unlabeled bovine IgM competed for binding of labeled IgM. This supports the role of the FcμR, and not the FαμR, in IgM binding. Finally, we showed that monocytes were able to ingest bacteria opsonized with serum deprived of IgG, indicating their ability to perform IgM-dependent phagocytosis. In conclusion, surface expression of FcμR by unstimulated blood leukocytes was demonstrated on B cells and monocytes, but not on T cells.