MAST-CELLS EXPRESS NOVEL CD8 MOLECULES THAT SELECTIVELY MODULATE MEDIATOR SECRETION

CD8, a marker largely restricted to subsets of T lymphocytes and NK ce lls, was detected on freshly isolated rat peritoneal mast cells (PMC). Using how cytometry, Percoll-enriched rat PMC (greater than or equal to 98% purity) were positive for the hinge region of CD8 alpha (67.5 /- 9.5%; Ab OX8) and CD8 beta (27.8 +/- 2.3%; Ab 341), CD8(+) PMC cons isted of two populations, CD8 alpha(+) (22.5%) and CD8 alpha(+)beta(+) (15.9%). Interestingly, G28, an Ab that identifies the IgV-like regio n of CD8 alpha on T lymphocytes, did not bind PMC, suggesting that PMC CD8 alpha is distinct from that on T lymphocytes. Moreover, a similar pattern of Ab positivity for CD8 was observed on a rat mast cell line , RBL 2H3. The presence of CD8 alpha immunoreactivity on rat PMC was f urther confirmed by confocal microscopy. In situ reverse-transcription PCR and reverse-transcription PCR analysis demonstrated that PMC cont ained mRNA transcripts encoding CD8 alpha. In functional studies of CD 8 on PMC, both TNF-alpha and nitric oxide production were induced by O X8 (CD8 alpha) and 341 Ab (CD8 beta) in a dose-dependent manner. Howev er, neither OX8 nor 341 induced histamine secretion from PMC, Ag-induc ed secretion of TNF-alpha, nitric oxide, and histamine was not affecte d by OX8 or 341 Abs, suggesting that there are distinct signaling mech anisms mediated by CD8 and Fc epsilon RI. These results indicate that rat PMC express functional CD8 molecules that may be distinct from tho se of T lymphocytes, The difference suggests there is a ligand other t han MHC class I for mast cell CD8.