MAC1 DISCRIMINATES UNUSUAL CD4(-)CD8(-) DOUBLE-NEGATIVE T-CELLS BEARING ALPHA-BETA ANTIGEN RECEPTOR FROM CONVENTIONAL ONES WITH EITHER CD4 OR CD8 IN MURINE LUNG

Pulmonary intraparenchymal leukocytes were purified from normal mice. By flow cytometry, 20-30% of the lymphocytes were positive for the exp ression of Mac1, a cell-surface antigen largely restricted to macropha ges, neutrophils and natural killer (NK) cells. Sorted Mac1(+) lung ly mphocytes were large and had abundant cytoplasm with few azurophilic g ranules. Because Mac1(+) lymphocytes did not contain any asiallo GM1() cells, they are not likely to be NK cells. By a two-color now cytome tric analysis, Mac1(+) lymphocytes were demonstrated to be TCR-alpha b eta(intermediate+), TCR-gamma delta(-), CD3(intermediate+), CD4(-), CD 8(-), Thy1(-), CD5(-), and B220(-). These Mac1(+) alpha beta T cells w ere not found in other organs such as spleen, thymus, liver, bone marr ow and intestine of mice uninfected and infected with Mycobacterium bo vis BCG. There was a considerable population of this unusual subset of alpha beta T cells in the lungs of congenitally athymic nude mice. In the Mac1(+) alpha beta T-cell population, the proportions of V beta 8 (+) T cells and of forbidden T-cell clones expressing V beta 6 TCR wer e not much different from that in the conventional T-cell population. These results indicated that extrathymically developed alpha beta T ce lls reside in considerable proportions in the lung and that Mac1 clear ly discriminates these cells from conventional ones. Interestingly, th e proportion of these cells increased in the lungs of mice infected wi th M. bovis BCG, which raises a possibility that these cells may play some role in the host defense against mycobacterial infection.