Cryptosporidium: cellular localization, structural analysis of absorptive cell-parasite membrane-membrane interactions in guinea pigs, and suggestion of protozoan transport by M cells.

In ilea of spontaneously infested guinea pigs, we examined the interface between the plasma membranes of cryptosporidia and absorptive cells using thin section and freeze fracture techniques. Initially, cryptosporidia invaginate microvilli, and the resulting redundant folds of membrane envelop the protozoan, thereby internalizing it in a membrane sac of host cell origin. Subsequently, a pentalaminar membrane fusion site develops at the base of the protozoan between the parasite's outer plasma membrane and the internalized host membrane. The membrane domains isolated by this fusion site are then modified: the host membrane disintegrates, and the isolated parasite membrane, which now directly contacts absorptive cell cytoplasm, becomes amplified. While cryptosporidia are restricted to the apex of absorptive cells, they may be found deep within the cytoplasm of M cells overlying Peyer's patches. Moreover, both intact and partially digested cryptosporidial organisms associate with macrophages subjacent to such M cells. These findings define the intracellular localization of cryptosporidia and suggest that cryptosporidial antigens may be sampled by intestinal lymphoid cells at sites underlying M cells.