A NOVEL 120-KDA ANTIGEN SHARED BY IMMATURE HUMAN THYMOCYTES AND LONG-TERM-ACTIVATED T-CELLS

In this study we report the characterization of monoclonal antibody (m Ab) 8B4/20, raised against immature human thymocytes, that identifies a novel leukocyte antigen. The molecular characterization of the antig en by immunoprecipitation and immunoblotting yields, under nonreducing conditions, a specific band of 120 kDa which, under reducing conditio ns, displays a slightly lower molecular mass (110 kDa). mAb 8B4/20 det ects a molecule found on the majority of thymocytes with an inverted g radient of expression when compared to CD3. It appears at high density on the CD3(-/low) thymocytes, at reduced density on the CD3(med) and double-positive thymocytes, and is absent on CD3(hi) and single-positi ve thymocytes and on peripheral blood T cells. Immunohistochemistry on frozen sections demonstrates cortical staining of the thymic lobules. Flow cytometric analysis of the different subsets of peripheral blood mononuclear cells shows that mAb 8B4/20 detects an antigen expressed only on CD56(+)/CD16(+) natural killer cells and on a fraction of CD14 (+) monocytes. T cells, B cells, erythrocytes, granulocytes and platel ets are consistently negative. The expression of the molecule on tumor cell lines does not show lineage restriction. Analysis of phytohemagg lutinin plus recombinant interleukin-2-activated peripheral blood lymp hocytes shows that mAb 8B4/20 identifies an antigen expressed on CD3() cells by week 3 of culture. Thus, it recognizes a very late activati on antigen (VLA) on mature T cells. The cell distribution and the elec trophoretic pattern of the molecule identified by mAb 8B4/20 is distin ct from that of known CD and of integrin/VLA molecules. Its function o n thymocytes is so far unknown; however, the binding of mAb 8B4/20 to tumor lines induces changes in the morphology and adhesive properties of the 8B4/20(+) cells growing in suspension. We suggest that mAb 8B4/ 20 recognizes a molecule that may be involved in interactions between thymocytes and other thymic structures that may be relevant for the se lection process.