Pneumococcal capsular polysaccharide-specific IgA triggers efficient neutrophil effector functions via FcalphaRI (CD89).

Specific anti-capsular polysaccharide IgG is believed to be important for protection against infection by Streptococcus pneumoniae. Significant IgA responses have been observed after vaccination with pneumococcal vaccines, but the role of this isotype in anti-pneumococcal host defense is unclear. Here, it is shown that purified serum IgA specific for pneumococcal capsular polysaccharides can initiate efficient cellular effector functions, such as phagocytosis, via interaction with the myeloid IgA receptor, FcalphaRI (CD89). The efficiency of FcalphaR-triggered granulocyte effector functions was comparable to that of FcgammaRIIa (CD32), as shown in experiments with bispecific antibodies. These results support a role for polysaccharide-specific IgA in antipneumococcal cellular effector function and suggest that FcalphaRI represents an important leukocyte receptor for immunity against S. pneumoniae.