The culture supernatants of LK1 cells, murine erythroleukemia cells, showed B cell-stimulating activity. Purification and NH2-terminal sequence analysis revealed that one of the candidates was murine IgE-dependent histamine-releasing factor (IgE-HRF), which is known to induce histamine from basophils. Recombinant IgE-HRF (rHRF) obtained from Escherichia coli- or 293-transformed embryonal kidney cells was tested for B cell-stimulating activity. Both rHRFs stimulated B cell proliferation in a dose-dependent manner. However, boiling or anti-HRF Ab abolished the B cell stimulatory effects of rHRF. Recombinant HRF showed strong synergistic effects with IL-2, IL-4, and IL-5 for B cell activation, with maximal activity in the presence of anti-CD40 Ab. Recombinant HRF increased MHC class II expression of B cells. It also increased Ig production from B cells. Treatment with polymyxin B, a neutralizing peptide antibiotic of LPS, did not reduce the activity of rHRF. In addition, FACS analysis using PE-conjugated rHRF showed that HRF bound to B cells. Recombinant HRF up-regulated the expression of IL-1 and IL-6 in B cells. In vivo administration of rHRF or the cDNA for rHRF increased total and Ag-specific Ig synthesis. Taken together, these results indicate that HRF stimulates B cell activation and function.

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