Mast cells express novel CD8 molecules that selectively modulate mediator secretion

CD8, a marker largely restricted to subsets of T lymphocytes and NK cells, was detected on freshly isolated rat peritoneal mast cells (PMC). Using flow cytometry, Percoll-enriched rat PMC (≥98% purity) were positive for the hinge region of CD8α (67.5 ± 9.5%; Ab OX8) and CD8β (27.8 ± 2.3%; Ab 341). CD8⁺ PMC consisted of two populations, CD8α⁺ β⁺ (15.9%). Interestingly, G28, an Ab that identifies the IgV-like region of CD8α on T lymphocytes, did not bind PMC, suggesting that PMC CD8α 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 CD8α immunoreactivity on rat PMC was further confirmed by confocal microscopy. In situ reverse-transcription PCR and reverse-transcription PCR analysis demonstrated that PMC contained mRNA transcripts encoding CD8α. In functional studies of CD8 on PMC, both TNF-α and nitric oxide production were induced by OX8 (CD8α) and 341 Ab (CD8β) in a dose-dependent manner. However, neither OX8 nor 341 Abs, suggesting that there are distinct signaling mechanisms mediated by CD8 and FcεRI. These results indicate that rat PMC express functional CD8 molecules that may be distinct from those of T lymphocytes. The difference suggests there is a ligand other than MHC class I for mast cell CD8.