The ocular secretory immune system of the rat

Ocular tissues and tears of adult male rats were examined for the presence of immunoglobulins (Ig) and secretory component (SC). By immunofluorescent analysis, we found the highest density of Ig-containing cells in the exorbital (lacrimal) gland. In contrast, few if any Ig-containing cells were observed in the conjunctiva or Harder's gland. IgA was the most frequent cell-associated isotype identified in the exorbital gland, and the number of IgA-containing cells was much greater than that of IgM- or IgG-containing cells. With respect to IgG, cell counts were fairly evenly distributed among the various subclasses. No IgE-containing cells were detected in ocular tissues. SC was identified exclusively within the acinar cells of the exorbital gland. In tears, IgA was the predominant isotype and occurred almost entirely in the polymeric form. Of the IgG subclasses, IgG 2a was present in the greatest concentration. Only trace amounts of IgG 2b were found, and IgG 1, IgG 2c and IgM could not be detected. The level of free SC in tears was relatively high, compared to the concentration of free SC reported in other mucosal secretions. These results suggest that the exorbital gland is the primary tissue associated with the ocular secretory immune system of the rat. Furthermore, our findings indicate that rat tears contain components that may provide immune defense for the ocular surface.