Bacterial transcytosis across conjunctival M cells.

PURPOSE Antigen-sampling M cells have been identified in conjunctival tissue overlying lymphoid follicles in rabbits and guinea pigs. Conjunctival M cells in the guinea pig display alpha(2-3) sialic acid on their surfaces, as evinced by selective labeling by Maackia amurensis leukoagglutinin (MAL)-I. Haemophilus influenzae strains OM12, which expresses the HMW1 adhesin for alpha(2-3) sialic acid, and Rd KW20, which lacks HMW1, were used to test the hypothesis that conjunctival M cells translocate large microbes. METHODS Fluorescein-labeled bacteria were instilled into the conjunctival sac for up to 130 minutes. Confocal laser scanning microscopy and electron microscopy were used to visualize bacterial distribution. RESULTS M cells, but not nonfollicular epithelial cells in the palpebral region, selectively bound and translocated bacteria. By 66 minutes, 423 +/- 165 bacteria/mm(2) of follicle-associated epithelial (FAE) surface were found in three-dimensional reconstructions extending 15.4 mum below the surface. By 127 minutes, the number of bacteria increased to 579 +/- 44/mm(2) of FAE surface and they had moved 50% deeper into the follicle. Coadministration with MAL-I reduced OM12 transport by 61%. Similarly, Rd KW20 uptake was 71% less at 63 minutes and 58% less at 121 minutes, indicating that OM12 uptake is at least partially mediated by binding to alpha(2-3) sialic acid. CONCLUSIONS Conjunctival M cells are a port of entry for large microbes and may play a role in initiation of mucosal immune responses against commensal or transient ocular bacterial species and may allow the entry of pathogens.