Colostral neutrophils express Fcα receptors (CD89) lacking γ chain association and mediate noninflammatory properties of secretory IgA

Abstract

Colostrum plays an important role in protecting newborn infants against acute gastrointestinal and respiratory infections. IgA antibodies have been considered the major effector component; however, the role of their receptors on colostral phagocytes, especially neutrophils, has not been studied. Here, we demonstrate that CD15+ colostrum neutrophils express IgA Fc receptors (FcαR, CD89) at levels similar to those of blood neutrophils. Most colostral cells (70%) bear secretory IgA (SIgA) on their surface (and intracellularly), whereas blood cells do not. The FcαR on colostral neutrophils was identified as the a.1 isoform with a similar molecular mass (55–75 kDa) as that identified for blood neutrophils. Removal of N-linked carbohydrates revealed a major protein core of 32 kDa for both cell types. In contrast, co-immunoprecipitation and immunoblot experiments using a mild detergent, digitonin, revealed a lack of γ chain association with FcαR (γ-less) exclusively on colostral neutrophils. The functional role of these γ-less FcαR cells was evaluated by measuring superoxide release and killing of SIgA-coated enteropathogenic E. coli. No increase in superoxide release was observed in colostral cells compared with blood neutrophils, whereas optimal release was obtained with PMA stimulation. Furthermore, despite similar bacterial phagocytosis index between both cell types, IgA-mediated bacterial-killing was not detectable with colostral neutrophils, whereas killing was detectable on blood cells. These results reveal exclusive expression of γ-less FcαR on colostral neutrophils associated with receptor hyperoccupation by IgA and with low, bacterial-killing activity, which suggest that this receptor may mediate noninflammatory effects of SIgA.