Dominance of conjunctiva-associated lymphoid tissue in the feline ocular immune system, with identification of lacrimal duct-associated lymphoid tissue.

Mucosa-associated lymphoid tissue, which plays a crucial role in both immunological protection and inflammation at mucosal sites, shows interspecies variation in its distribution. In the ocular region, lacrimal duct-associated lymphoid tissue (LDALT) has rarely been investigated in animals other than humans or rodents. Although conjunctiva-associated lymphoid tissue (CALT) has been reported in various species, its cell composition remains unclear. Herein, we aimed to elucidate the morphology and development of the feline ocular immune system by examining the lacrimal tract and its associated tissues in adolescent (7- to 8-month-old) and fetal cats. The feline lacrimal tract, composed of the lacrimal canaliculi, lacrimal sac, and nasolacrimal duct, was visualized using CT. Histologically, a small lymphocyte aggregate, identified as LDALT, was observed exclusively along the lacrimal canaliculus in one of the four adolescent cats. In contrast, extensive development of the CALT was observed in tissues surrounding the openings of the lacrimal tract, particularly on the cornea-facing side of the third eyelid. CALT is characterized by CD20(+) B cell aggregates surrounded by CD3(+) T cells, including a small subset coexpressing CD20 and CD3. Neither LDALT nor CALT was observed in late gestational fetuses. In conclusion, CALT appears to contribute more significantly than LDALT to feline ocular immunity. Furthermore, given the pro-inflammatory nature of both CALT and CD3(+)CD20(+) cells, our findings suggest that CALT is involved in the pathology of feline conjunctivitis.