A recombinant fragment of human surfactant protein D suppresses basophil activation and T-Helper type 2 and B-cell responses in grass pollen-induced allergic inflammation

Rationale: Recombinant fragment of human surfactant protein D (rfhSP-D) has been shown to suppress house dust mite- and Aspergillus fumigatus-induced allergic inflammation in murine models. Objectives: We sought to elucidate the effect of rfhSP-D on high-affinity IgE receptor- and CD23-mediated, grass pollen-induced allergic inflammatory responses. Methods: rfhSP-D, containing homotrimeric neck and lectin domains, was expressed in Escherichia coli BL21(lDE3)pLysS cells. Peripheral blood mononuclear cells and sera were obtained from individuals with grass pollen allergy (n = 27). The effect of rfhSP-D on basophil activation and histamine release was measured by flow cytometry. IgE-facilitated allergen binding and presentation were assessed by flow cytometry. T-helper cell type 2 (Th2) cytokines were measured in cell culture supernatants. The effect of rfhSP-D on IgE production by B cells when stimulated with CD40L, IL-4, and IL-21 was also determined. Measurements and Main Results: rfhSP-D bound to Phleum pratense in a dose- and calcium-dependent manner. Allergen-induced basophil responsiveness and histamine release were inhibited in the presence of rfhSP-D, as measured by CD63, CD203c (P = 0.0086, P = 0.04205), and intracellularly labeled diamine oxidase (P = 0.0003, P = 0.0148). The binding of allergen-IgE complexes to B cells was reduced by 51% (P = 0.002) in the presence of rfhSP-D. This decrease was concomitant with reduction in CD23 expression on B cells (P, 0.001). rfhSP-D suppressed allergen-driven CD27²CD4¹CRTh2¹ T-cell proliferation (P, 0.01), IL-4, and IL-5 levels (all P, 0.01). Moreover, rfhSP-D inhibited CD40L/IL-4-and IL-21-mediated IgE production (77.12%; P = 0.02) by B cells. Conclusions: For the first time, to our knowledge, we show that rfhSP-D inhibited allergen-induced basophil responses at a single-cell level and suppressed CD23-mediated facilitated allergen presentation and Th2 cytokine production. In addition, rfhSP-D inhibited IgE synthesis by B cells, which is also a novel observation.