Internalization of P. carinii by alveolar macrophages collected from macrophage MR-deficient (MR −/− ) mice. Alveolar macrophages were isolated from 6–8-wk-old, male C57BL/6 or MR −/− mice and pretreated with 2.0 μg/ml 2A11 (gray bars) or isotype control antibody (solid bars). Alexa Fluor 488 succinimidyl ester–labeled P. carinii cysts and trophozoites were added at a macrophage to P. carinii ratio of 1:5 in an in vitro phagocytosis assay. Thereafter, aliquots of macrophage/ P. carinii cocultures were subjected to confocal microscopy for determination of P. carinii internalization. *, significant differences between cocultures of isotype and 2A11 (P