T90/44 (9.3 antigen). A cell surface molecule with a function in human T cell activation

T90/44 is a cell surface antigen which is present on human T cells of the helper and cytotoxic subsets and which binds the 9.3 monoclonal antibody (9.3 mAb). It is expressed in the form of 90-kDa disulfide-bonded dimers of a 44-kDa polypeptide and of free 44-kDa subunits. The function of T90/44 was investigated in a series of T cell function assays. 9.3 mAb was found to inhibit the activation of class II-restricted cloned T helper cells derived from leprosy patients and reactive with M. leprae antigens. The inhibition was first found at 1-10 ng/ml 9.3 mAb and regularly increased with the antibody concentration. The extent of the inhibition varied among different T cell clones in proportion to the respective different levels of T90/44 expression at their cell surface. The proliferative responses of peripheral blood lymphocytes (PBL) to purified protein derivative of M. tuberculosis (PPD) and tetanus toxoid were enhanced by the 9.3 mAb resulting in up to 20-30-fold increase of 3H-thymidine incorporation. After phytohemagglutinin-induced activation of PBL, the number of T90/44 molecules per cell expressed at the cell surface rose from day 0 to day 7 by a factor of about 10. High concentrations of 9.3 mAb (5-10 micrograms/ml) at low cell densities and in the presence of monocytes in culture media supplemented by fetal calf serum were directly mitogenic for resting lymphocytes. The cytolytic effector functions of class I-restricted cytotoxic T lymphocytes (CTL) were not modulated by 9.3 mAb. The mixed lymphocyte reactions of three class I-restricted CTL to their specific target cells were found not to be significantly influenced by 9.3 mAb. In conclusion it is proposed that an antigen-independent T cell activation pathway can be entered at T90/44.