SummaryWe studied the humoral response of mice immunized with soluble CD4-rgp120 complex, testing polyclonal and monoclonal antibodies (mAbs) with the aim of identifying molecular changes that take place after the first interaction between human immunodeficiency virus and the cell surface. The antisera had a paradoxically high syncytia-blocking titer associated with anti-CD4 specificity, while their capacity to inhibit CD4-gp120 binding was relatively modest . One of the mAbs produced from these responders blocks syncytia formation but does not inhibit CD4 interaction with gp120. Apparently, this mAb interacts with the CD4 moiety of CD4-gp120 complex and prevents a post-binding event necessary for membrane fusion and viral infection .

In this study we describe the generation and characterization of interspecies somatic cell hybrids between human activated mature T cells and mouse BW5147 thymoma cells. A preferential segregation of human chromosomes was observed in the hybrids. Phenotypic analysis of two hybrids and their clones demonstrated coexpression of CD4 and CD69 antigens in the same cells. Segregation analysis of an informative family of hybrids followed by molecular and karyotype studies clearly demonstrated that the locus encoding CD69 antigen mapped to human chromosome 12. Although the expression of CD69 antigen is an early event after T-lymphocyte activation and rapidly declines in absence of exogenous stimuli, in the hybrids described in this study the expression was constitutive, similarly to what was previously found in early thymocyte precursors and mature thymocytes. In this respect it was important to note that the behavior of the hybrids in culture strongly suggested a dominant influence of the thymus-derived mouse tumor cell genome in controlling the constitutive expression of human CD69. These hybrids may thus provide a system to study the genetic and molecular mechanisms controlling the expression and function of this activation antigen.

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