Pneumococcal capsular polysaccharide-specific IgA triggers efficient neutrophil effector functions via FcαRI (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, Fc alpha RI (CD89), The efficiency of Fc alpha R-triggered granulocyte effector functions was comparable to that of Fc gamma RIIa (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 Fc alpha RI represents an important leukocyte receptor for immunity against S. pneumoniae.