Molecular identification of IgE-dependent histamine-releasing factor as a B cell growth factor

The culture supernatants of LK1 cells, murine erythroleukemia cells, showed B cell-stimulating activity. Purification and NH2-terminal sequence analys is revealed that one of the candidates was murine IgE-dependent histamine-r eleasing factor (IgE-HRF), which is known to induce histamine from basophil s. Recombinant IgE-HRF (rHRF) obtained from Escherichia coli- or 293-transf ormed embryonal kidney cells was tested for B cell-stimulating activity. Bo th rHRFs stimulated B cell proliferation in a dose-dependent manner. Howeve r, 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 neutral izing peptide antibiotic of LPS, did not reduce the activity of rHRF. In ad dition, FACS analysis using PE-conjugated rHRF showed that HRF bound to B c ells. Recombinant HRF up-regulated the expression of IL-1 and IL-6 in B cel ls. 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.