Involvement of T44 molecules in an antigen-independent pathway of T cell activation. Analysis of the correlations to the T cell antigen-receptor complex.

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Abstract
Prior studies indicate that the 9.3 monoclonal antibody (mAb) which defines a 44 kD T lineage-specific glycoprotein (T44) enhances the proliferative response of peripheral blood T lymphocytes to phytohemagglutinin (PHA) or allogeneic cells. The T44 molecule was expressed in both resting and activated T lymphocytes and in a subset of thymocytes, as assessed by indirect immunofluorescence and flow cytofluorometry. In view of the potential importance of T44 in T cell activation, we investigated the ability of the 9.3 (anti-T44) antibody to stimulate [human] peripheral blood T lymphocytes under culture conditions giving optimal proliferative responses of purified T cells, provided that adherent cells were added to the culture. Maximal proliferation in response to 9.3 antibody was consistently detected at day 5 (at day 3 with anti-T3 or PHA). Moreover, triggering of T lymphocytes with 9.3 antibody (in the presence of adherent cells) resulted in strong IL-2 production that peaked at 48 h. Analysis of the physical and functional relationship between the T44 molecule and other molecules involved in T cell activation, including the clonotypically restricted Ti and the monomorphic T3 or T11 molecules, was carried out on a mutagenized Jurkat T leukemia cell line. This mutant, termed JA3 (surface phenotype: T11+, T3+, 3A1+, T4-, T8-, DR-, Tac-, 4F2+, T44+) produced large amounts of IL-2 upon stimulation with PHA, anti-T3, or anticlonotypic mAb in conjunction with phorbol myristate acetate (or adherent cells). The molecules precipitated by anti-T44 mAb from 125I-labeled JA3 cells appeared as a diffuse band of Mr 40-45,000 under reducing conditions; under nonreducing conditions, a prominent band of Mr 80-85,000 was observed, while the Mr 40-45,000 band was greatly reduced. Thus, T44 molecules in both reducing and nonreducing conditions had relative molecular weights similar to that of molecules carrying clonotypic (Ti) determinants. In addition, like anti-Ti or anti-T3 mAb, anti-T44 antibody induced JA3 cells to produce large amounts of IL-2 in the presence of phorbol myristate acetate. Other similarities between T44 and molecules carrying clonotypic structures included the susceptibility to antibody-induced modulation and the late reexpression (72 h) at the cell surface after modulation. Taken together, these experiments suggest that anti-T44 mAb might recognize a monomorphic determinant of the T cell receptor molecule or be physically or functionally linked to the T3-Ti complex. However, antibody-induced modulation of T44 molecules did not lead to the loss of T3, Ti, or T11 molecules, nor did modulation of T3 or Ti affect the expression of T44 antigen at the JA3 cell surface. In addition, sequential immunoprecipitation experiments showed that preclearing of JA3 cell lysates with anti-T44 antibody did not remove molecules carrying clonotypic structures, and that preclearing with anticlonotypic mAb did not affect subsequent precipitation with anti-T44 mAb. Thus, the antigen-independent, T44-mediated mode of T cell triggering is distinct from that initiated by the T3-Ti antigen receptor complex or by T11 molecules. However, modulation of the T3-Ti receptor complex, although it had no effect on T44 expression, did regulate the ability of T44 molecules to induce IL-2 release. The nature of the natural ligand of T44 molecule and the role in vivo of T44-triggered pathway of human T cell activation remains to be determined.