The human recombinant histamine releasing factor: Functional evidence thatit does not bind to the IgE molecule

Background: We have previously shown that the human recombinant histamine r eleasing Factor (HrHRF) caused histamine release From a subset of basophils from donors with allergy, and this release seemed to be dependent on the p resence of a certain type of IgE, termed IgE+. IgE molecules that did not s upport HrHRF-induced histamine release were termed IgE-. However, subsequen tly we demonstrated that HrHRF primes anti-IgE-antibody-induced histamine r elease from all basophils, irrespective of the type of IgE on the cell surf ace. Objective: Because these data suggested that HrHRF does not exert its biolo gic effects by binding to IgE, but rather that it interacted with a surface receptor on the basophil, me wanted to obtain functional evidence that HrH RF did or did not bind to the IgE molecule. Methods: The rat basophilic leukemia cell fine (RBL-SX38), which has been t ransfected to express a functional human Fc epsilon RI (alpha-, beta-, and gamma-chains of the receptor) in addition to the normal rat FceRI, was used . The presence of the human Fc epsilon RI receptor enables these cells to b e sensitized with human IgE, Cells were passively sensitized with 1000 ng/m L human IgE+ or 1000 ng/mL human Ige for 60 minutes at 37 degrees C. Unsens itized cells served as a control. After the cells were washed, 1 x 10(5) ce lls were stimulated in the presence of 1 mmol/L Ca2+ with 0.1 mu g/mL anti- IgE, 40 mu g/mL HrHRF, or 40 mu g/mL mouse recombinant HRP (MrHRF), which h as 96% homology to HrHRF, Results: Mean anti-IgE-induced histamine release was 33% +/- 15%, and there was no difference between IgE+ sensitization (32% +/- 12%) and IgE- sensit ization (34% +/- 18%). However, in contrast to human basophil experiments, neither HrHRF (0% +/- 0%) nor MrHRF (3% +/- 5%) caused histamine release in RBL cells sensitized with IgE+, In addition, priming the transfected RBL-S X38 cells or the parental cell line, RBL-2H3 cells, with HrHRF or MrHRF did not increase anti-IgE-induced histamine release. Conclusion: The results indicate that HrHRF does not bind to IgE, either Ig E+ or IgG. Therefore it appears likely that rHRF signals through its own sp ecific receptor, which is not expressed or functional on RBL-SX38 or RBL-2H 3 cells, but which seems to be expressed on basophils of atopic and nonatop ic donors.