Characterization of lymphocyte subsets over a 24-hour period in Pineal-Associated Lymphoid Tissue (PALT) in the chicken

BACKGROUND: Homeostatic trafficking of lymphocytes in the brain has important relevance to the understanding of CNS disease processes. The pineal gland of the chicken contains large accumulations of lymphocytes that suggest an important role related to homeostatic circadian neuro-immune interactions. The purpose of this initial study was to characterize the lymphocyte subsets in the pineal gland and quantitate the distribution and frequency of lymphocyte phenotypes at two time points over the 24-hour light:dark cycle. RESULTS: PALT comprised approximately 10% of the total pineal area. Image analysis of immunocytochemically stained sections showed that the majority of lymphocytes were CD3(+ )(80%) with the remaining 20% comprising B-cells and monocytes (Bu-1(+)), which tended to distribute along the periphery of the PALT. T-cell subsets in PALT included CD4(+ )(75–80%), CD8(+ )(20–25%), TCRαβ/Vβ(1)(+ )(60%), and TCRγδ(+ )(15%). All of the T-cell phenotypes were commonly found within the interfollicular septa and follicles of the pineal gland. However, the ratios of CD8(+)/CD4(+ )and TCRγδ(+)/TCRαβ/Vβ(1)(+ )within the pineal tissue were each 1:1, in contrast to the PALT where the ratios of CD8(+)/CD4(+ )and TCRγδ(+)/TCRαβ/Vβ(1)(+ )each approximated 1:4. Bu-1(+ )cells were only rarely seen in the pineal interstitial spaces, but ramified Bu-1(+ )microglia/macrophages were common in the pineal follicles. Effects of the 24-h light:dark cycle on these lymphocyte-pineal interactions were suggested by an increase in the area of PALT, a decline in the density of TCRαβ/Vβ(1)(+ )cells, and a decline in the area density of Bu-1(+ )microglia at the light:dark interphase (1900 h) compared to the dark:light interphase (0700 h). CONCLUSION: The degree of lymphocyte infiltration in the pineal suggests novel mechanisms of neuro-immune interactions in this part of the brain. Our results further suggest that these interactions have a temporal component related to the 24-hour light:dark cycle and that CD8(+ )and TCRγδ(+ )T-cells are preferentially recruited to the pineal follicles. Pineal microglia/macrophages were common and represent an important candidate for mediating these lymphocyte-pineal interactions via secretion of cytokines and chemokines.