Characterization of a novel human surface molecule selectively expressed by mature thymocytes, activated T cells and subsets of T cell lymphomas

We have previously characterized mouse H4 (mH4), a surface glycoprotein rec ognized by the C398.4A monoclonal antibody. We now show that C398.4A also b inds its human putative homolog (hpH4). Both hpH4 and mH4 (1) are selective ly expressed by activated T cells and mature thymocytes, (2) are disulfide- linked dimers of two chains (29/37 kDa in humans, 25/29 kDa in mice), whose N-deglycosylation produces a single band at 20-21 kDa, and (3) display a l ow association with CD4 and the TCR. The expression pattern of hpH4 and its biochemical features showed that it is different from other known activati on molecules, and this was confirmed when analysis of the tryptic digest of the hpH4 29-kDa band by peptide mass searching using matrix-assisted laser desorption ionization mass spectrometry did not reveal any significant hom ology with other molecules. In normal lymphoid tissue, hpH4 is expressed by T cells located at the periphery of lymph node germinal centers and paraco rtical areas. In T cell neoplasia, expression of hpH4 clusters with a subse t of peripheral T cell lymphomas with a large-cell component, and with case s of angioimmunoblastic T cell lymphomas. Overall, these data provide evide nce for a novel T cell activation molecule that could help in the phenotypi c categorization of T cell malignancies.