Developmental changes in conjunctiva-associated lymphoid tissue of the rabbit

Organized conjunctiva-associated lymphoid tissue (O-CALT) is constantly exposed to environmental antigens and plays a central role in the common mucosal immune system. This study was undertaken to investigate whether O-CALT in rabbit lymphoid tissue, as in other lymphoid tissue in other mammals, changes with age.

Fluorescence stereomicroscopy was used to measure the number and size of conjunctival follicles stained with propidium iodide in rabbits ranging in age from 2 days to 57 months. To assess the function of M cells, an antigen-sampling cell type found in the follicle-associated epithelium, transcytosis of fluorescent latex beads was evaluated with confocal microscopy.

O-CALT was not present in rabbits at birth, but appeared less than 24 hours after eyes opened at approximately day 11. The number of follicles increased with age until adolescence (2-4 months), when the number stabilized through early adulthood (17-20 months). In aged rabbits (47-57 months), there was a dramatic decline in the number of follicles. This disappearance was most pronounced in the superior conjunctiva. Average follicle diameter increased with age, except in the superior conjunctiva of aged rabbits, where the few remaining follicles were generally smaller. The uptake of latex beads showed that M-cell function was similar in all age groups.

Age-related changes in rabbit O-CALT are similar to those that have been reported for the human conjunctiva. Preferential uptake of latex beads by follicle-associated epithelium indicates that the presence and function of M cells are not affected by aging. The lower level of O-CALT in young and elderly animals, however, would be expected to decrease their ocular mucosal immune responses.