We previously characterized a dimeric, Mr = 115,000, developmentally regulated mouse T cell-activating molecule (THAM). We show in this report that the THAM-specific mAb H194-112 exhibits strong reactivity with several nonlymphoid tissues including polarized enterocytes from small intestine, kidney cortical tubuli, and liver bile ductuli, as well as kidney glomeruli and lung alveoloar pneumocytes. Both the tissue distribution and the structural features of THAM made it likely that this molecule belongs to the ectoenzyme family. This was confirmed by the following experimental evidence: 1) the H194-112+ molecules from enterocyte brush borders (BB) or M14.T
thymoma cells were recognized by several
antisera specific for intestinal
aminopeptidase N (AP-N); 2) mAb H194-112 was found to immunodeplete the AP-N activity from both thymic or enterocyte BB
detergent extracts; 3) the hydrophilic, Mr = 115,000 form obtained by
papain treatment of
thymoma or enterocyte BB could be immunopurified on H194-112 column and exhibited, after hypotonic elution, strong enzymatic activity on the AP-N substrates (i.e., leucyl or alanyl beta-derivatives); 4) mAb H194-112 was found to inhibit the AP-N activity when assayed on alanyl but not leucyl
beta-naphthylamide substrate; and 5) preincubation of AP-N with mAb H194-112 prevented the inhibiting effects of
bestatin and D,L-methionyl hydroxamate on AP-N activity. These data add a new member to the list of functional ectoenzymatic markers of lymphoid cells (i.e., CD10, CD13, CD26, CD55, and CD73). In view of the known immunomodulating properties of
bestatin, one may speculate that the T cell-activating effects of mAb H194-112 is related to an impairment of a surface enzymatic function regulating lymphoid cell activation.