Rat monocytes up-regulate NKR-P1A and down-modulate CD4 and CD43 during activation in vivo: monocyte subpopulations in normal and IFN-gamma-treated rats.

LEW rats were treated intravenously with recombinant rat interferon-gamma (IFN-gamma) for 3 days to achieve intravascular accumulation, proliferation, and activation of monocytes. Monocytes, defined by their expression of the ED1, ED9, and Ox41 antigens, were recovered from the vasculature by perfusion with PBS/EDTA, subsequently depleted of erythrocytes and granulocytes by Percoll density gradient centrifugation, and analyzed by flow cytometry and immunocytology. In untreated and control-infused specified pathogen-free (SPF) rats, lymphocytes and monocytes formed overlapping cell populations with respect to size and internal granularity. At least two intravascular monocyte subsets, probably central and marginating cells, were distinguished by their size and differential expression of CD43, CD4, CD11a, CD18, and L-selectin. It is interesting to note that a fraction of the monocytes in normal and control-infused animals carried the NKR-P1A molecule. IFN-gamma treatment provoked a duplication of monocyte size and granularity. Both the number of positive monocytes and the level of expression of NKR-P1A strongly increased after IFN-gamma infusion, whereas CD43 (leukosialin) and CD4 were impressively down-regulated. NKR-P1A+ L-selectin+ CD43low CD4- monocytes also occur in the vasculature of rats during immune reactions in vivo. We speculate that these cells are involved in organ damage and that their number is controlled by activation-induced cell death within the vessels.