Early lymphocyte activation molecule defined by the monoclonal antibody MLR-3: biochemical and functional studies

The MLR-3 monoclonal antibody reacts with activated but not with resting lymphocytes. We report that MLR-3 identifies an early activation molecule since its binding is detectable on T cells 1.5-2 hr after in vitro activation. Its expression, therefore, does not require DNA synthesis and precedes, by many hours, that of the receptors for interleukin-2 (IL-2R) and transferrin (TF-R). The MLR-3 antigen is also found on activated thymocytes (including the large early thymic CD3- subset) and B cells. The majority of T- and B-lymphoblastoid cell lines, as well as the myeloid and erythroid cell lines HL60, GM1 and K562, are MLR-3+; conversely, non-haemopoietic cell lines are MLR-3 negative. Seventy percent of B-cell chronic lymphocytic leukaemia and 15% of B non-Hodgkin's lymphomas (B-NHL) are MLR-3+. On tissue sections MLR-3 is reactive with epithelia, sweat glands, hair follicles and Henle's loop but not with vessels, connective, endothelium and many other tissues. In vitro studies show that MLR-3 (1-100 micrograms/ml) significantly alters the thymidine uptake of mitogen-treated lymphocytes:augmentation is found when T and B cells are induced with TPA-Ionomycin and reduction when induced with phytohaemoagglutinin (PHA) or Staphylococcus aureus Cowan strain 1 (SAC), respectively. On SDS-PAGE, MLR-3 immunoprecipitates a disulphide-linked heterodimer of MW 29,000-35,000: both subunits are glycosylated, phosphorylated and exhibit a pI of 4.1 and 5.0, respectively. Our data, particularly the in vitro results, suggest that the MRL-3 molecule could have an important role in the early hours of activation for the progression of resting lymphocytes into mitosis.