The IA4 mAb was identified among a series of antibodies raised in BALB/c mice after immunization against a HLA class I-deficient, lymphokine-activated killer (LAK)-susceptible EBV-B lymphocyte line. The IA4 antibody was selected because of its high expression, in the range of 10(5) to 25 x 10(5) sites/cell, on several B lymphocyte lines (EBV-transformed or Burkitt) and monocytic lines such as HL60 and U937, and because its expression was correlated with both target susceptibility to LAK lysis and reduced expression of HLA class I surface Ag on two pairs of EBV-B-transformed cell lines (721/721.134 and MM/10F2). Despite the strategy followed to raise the mAb and the correlation mentioned above, no direct role of the IA4 molecules in LAK susceptibility has been established, since the IA4 molecule is poorly expressed on the sensitive targets Daudi and K562; moreover, the IA4 antibody did not affect reproducibly the in vitro killing of positive target cells by LAK effectors. The IA4 antibody was poorly immunoprecipitating and the surface molecule recognized was identified by gene cloning following an expression strategy using a U937 cDNA library transfected in COS cells, and a screening strategy based on membrane expression of IA4 molecule. The IA4 cDNA is virtually identical to "R2," a mRNA species previously identified in activated human T cells by subtractive hybridization. The IA4 cDNA contains an open reading frame coding for a protein 267 amino acids long with four potential transmembrane domains and one large external hydrophilic domain of about 110 amino acids, possibly glycosylated. The encoded protein belongs to a family of surface molecules, the tetra spans transmembrane protein superfamily, all displaying the four transmembrane domains, expressed on various cell types including lymphocytes (CD9, CD37, CD53, TAPA-1), melanoma cells (ME491), and intestinal cells (CO-029). These molecules have been reported to be involved in cell activation and cell death. Surprisingly, the Schistosoma mansoni Ag Sm23 displays significant homologies with this family. The IA4 molecule is a widely distributed surface marker expressed on circulating lymphocytes and monocytes, newborn thymocytes, and the cell lines mentioned above. The IA4 molecule expression is up-regulated upon cell activation. Weakly expressed on resting peripheral T and B lymphocytes and large granular lymphocytes (NK), its expression roughly doubles after activation by PHA, staphylococcus aureus Cowan I, and IL-2, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

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