Protein Fv produced during vital hepatitis is a novel activator of human basophils and mast cells.

Protein Fv is found in the normal liver and is released in the stools of patients suffering from viral hepatitis. Protein Fv isolated from five patients stimulated the release of histamine and sulfidopeptide leukotriene C4 from purified and unpurified peripheral blood basophils. Protein Fv absorbed with protein A-Sepharose coated with polyclonal IgG did not induce histamine secretion, whereas removal of putative contaminating Ig did not modify the releasing activity. The characteristics of the release reaction were similar to those of rabbit IgG anti-Fc fragment of human IgE (anti-IgE). There was an excellent correlation (Spearman rank coefficient (rs) = 0.83; p < 0.001) between the maximal percent histamine release induced by protein Fv and that induced by anti-IgE from basophils. Preincubation of basophils with either protein Fv or anti-IgE resulted in complete cross-desensitization to a subsequent challenge with heterologous stimulus. Basophils from which IgE had been dissociated by brief exposure to lactic acid no longer released histamine in response to anti-IgE and protein Fv. A monoclonal IgE purified from a myeloma patient (patient ADZ) blocked both anti-IgE- and protein Fv-induced releases, whereas human polyclonal IgG and a monoclonal IgG purified from another myeloma patient (patient ZEG) selectively blocked protein Fv-induced secretion. Protein Fv also induced the release of preformed (histamine and tryptase) and de novo synthesized mediators (sulfidopeptide leukotriene C4 and/or PGD2) from mast cells purified from human lung parenchyma and skin tissues. There was a significant correlation between the maximal percent histamine release induced by protein Fv and anti-IgE from skin mast cells (rs = 0.63; p < 0.01). There was also an excellent correlation between histamine and tryptase release caused by protein Fv from both lung (rs = 0.80; p < 0.001) and skin mast cells (rs = 0.70; p < 0.01). Thus, we established that protein Fv acts as a novel activator of human basophils and mast cells presumably by interacting with the VH domain of the IgE.