Sialic acid–binding immunoglobulin-like lectin-3 (Siglec-3 [CD33]) is a major Siglec expressed on human mast cells and basophils; engagement of CD33 leads to inhibition of cellular signaling via immunoreceptor tyrosine–based inhibitory motifs.

We sought to inhibit human basophil degranulation by simultaneously recruiting inhibitory CD33 to the IgE-FcεRI complex by using monoclonal anti-IgE directly conjugated to CD33 ligand (CD33L).

Direct and indirect basophil activation tests (BATs) were used to assess both antigen-specific (peanut) and antigen-nonspecific (polyclonal anti-IgE) stimulation. Whole blood from donors with allergy was used for direct BAT, whereas blood from donors with nonfood allergy was passively sensitized with plasma from donors with peanut allergy in the indirect BAT. Blood was incubated with anti-IgE–CD33L or controls for 1 hour or overnight and then stimulated with peanut, polyclonal anti-IgE, or N-formylmethionyl-leucyl-phenylalanine for 30 minutes. Degranulation was determined by measuring CD63 expression on the basophil surface by flow cytometry.

Incubation for 1 hour with anti-IgE–CD33L significantly reduced basophil degranulation after both allergen-induced (peanut) and polyclonal anti-IgE stimulation, with further suppression after overnight incubation with anti-IgE–CD33L. As expected, anti-IgE–CD33L did not block basophil degranulation due to N-formylmethionyl-leucyl-phenylalanine, providing evidence that this inhibition is IgE pathway–specific. Finally, CD33L is necessary for this suppression, as monoclonal anti-IgE without CD33L was unable to reduce basophil degranulation.

Pretreating human basophils with anti-IgE–CD33L significantly suppressed basophil degranulation through the IgE-FcεRI complex. The ability to abrogate IgE-mediated basophil degranulation is of particular interest, as treatment with anti-IgE–CD33L before antigen exposure could have broad implications for the treatment of food, drug, and environmental allergies.