We have recently described a polymorphic Fc receptor for murine IgG2b (mIgG2b), present on human monocytes and EBV-transformed B lymphocytes. The present study shows that anti-HLA class II monoclonal antibody (MoAb) completely inhibits both the (Fc receptor-dependent) T-cell proliferation, induced by mIgG2b anti-CD3 MoAb, and rosetting with mIgG2b-sensitized erythrocytes. This inhibition is also observed with F(ab')2 fragments of anti-HLA class II MoAb, and is therefore not Fc mediated. The Fc receptor for mIgG2b is also present on EBV-transformed B cells obtained from a patient with 'bare lymphocyte syndrome', that completely lack HLA class II antigens. Therefore, the Fc receptor for mIgG2b and HLA class II antigens are not identical. Since the low affinity receptor for IgE (Fc epsilon II; CD23) was reported to be associated at the cell surface with HLA class II antigens, we have compared both types of Fc receptor, and observed that human IgE strongly inhibits the mitogenic effect of murine IgE anti-CD3 but not of mIgG2b anti-CD3 MoAb. We conclude that the human Fc receptor for mIgG2b is strongly inhibited by anti-HLA class II MoAb, but is not identical to HLA class II or Fc epsilon RII.

We have previously described a polymorphic human Fc receptor for murine IgG2b (mIgG2b). This receptor was defined by the binding of (complexed) mIgG2b to monocytes and Epstein-Barr virus (EBV)-transformed B cells. Three per cent of normal individuals were high responders with respect to mIgG2b (mIgG2b-HR), whereas the other individuals were low responders for mIgG2b (mIgG2b-LR). In the present study we investigated the effect of proteolytic enzymes on the Fc-mediated binding of mIgG2b to EBV-B cells. Pronase, human leucocyte elastase and cathepsin G caused an increased binding (in an EA-rosetting assay) of mIgG2b to EBV-B cells from mIgG2b-HR, but not from mIgG2b-LR. Simultaneous immunofluorescence studies demonstrated that these proteolytic enzymes strongly reduced the expression of Fc gamma RII and Fc epsilon RII on these cells, whereas HLA class I or HLA class II molecules were not affected. These findings strongly suggest that binding of mIgG2b is not mediated by Fc gamma RII or Fc epsilon RII. We also studied the effect of proteolysis on mIgG2b-HR EBV-B cells from an HLA class II-negative individual. In this case EA-mIgG2b rosetting was decreased after proteolysis, suggesting that HLA class II molecules may have a role in protecting the binding site for mIgG2b against proteolytic destruction.

Graft-versus-host disease (GVHD) after allogeneic bone marrow transplantation (BMT) is initiated by immunocompetent T cells present in the graft. Selective elimination of distinct T-cell subsets or a sufficient, but not complete T-cell depletion, might abolish severe GVHD without graft rejection and loss of the anti-tumour potential. In this study we analysed the efficacy of different monoclonal antibodies (MoAb) WT32 (CD3), OKT4 (CD4), T101 (CD5), WT1 (CD7), and WT82 (CD8) with respect to their cytotoxicity to T cells either as immunotoxin (IT) or in combination with complement. The cytotoxic potential was assessed by protein synthesis inhibition and clonogenic assays. The ricin A conjugated MoAb exerted only a minor effect on blood or bone marrow T cells, although they were highly inhibitory to T-cell lines. However, in the presence of 20 mM ammonium chloride, IT directed against CD3, CD5, and CD7 were highly cytotoxic. IT directed against CD4 and CD8 were less effective, due to a low internalization. The complement-mediated cytotoxicity was efficient for all antigens used. The natural killer (NK) activity, as measured by cytotoxicity to K562, was hardly depressed by anti-CD3, anti-CD4, anti-CD5, and anti-CD8, but was eliminated by anti-CD7. All procedures used had only a minimal effect on haematopoietic progenitors as measured by CFU-GM and BFU-E assays. We concluded that, although the T-cell population can be eliminated with the combination of anti-CD3, anti-CD5, and anti-CD7 antibodies plus complement, IT with 20 mM NH4Cl appear to kill higher amounts of T cells. Selective elimination of CD4- and CD8-positive cells is effectively obtained by MoAb with complement.

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