7th International AIDS Conference Florence, Italy — June 16-21, 1991

Int Conf AIDS 1991 Jun 16-21; 7:93 (abstract no. M.A.1004)
Barton G, Sorio C, Cassatella MA, Rossi F; Institute of General Pathology, University of Verona, Italy

OBJECTIVE: Since the significance of CD4 expression by human mononuclear phagocytes is largely unknown, we initiated studies aimed to establish whether CD4 expressed by human myeloid cell lines and macrophages is able to deliver a signal. Furthermore we have investigated regulation of CD4 expression on human myeloid cell lines and macrophages.

METHODS: The association of CD4 with a tyrosin kinase activity has been studied by performing in vitro kinase assays on anti-CD4 immunoprecipitates from lysates of U937 and ML3 cells, and monocyte-derived macrophages. CD4 expression was studied by cytofluorografic analysis; CD4 mRNA expression was studied by Northern blotting analysis.

RESULTS: In vitro kinase assays performed on anti-CD4 immunoprecipitates from lysates of U937 and ML3 cells, and monocyte-derived macrophages (MDM) demonstrated that, also in these cells, CD4 is associated with a kinase activity able to phosphorylate enolase, and a polypeptide of about 56 kDa present in the immunoprecipitate. Immunoprecipitates done with anti-B2 chain of leukocyte integrins Abs, from lysates of U937 cells, or with anti-CD4 Abs, from neutrophils or eosinophils, did not display any kinase activity. This observation indicated that, as well established for T cells, also in myeloid cells CD4 is associated to a putative tyrosin kinase able to phosphorylate itself. We are pursuing attempts to identify the aminoacid phosphorylated in the 56 kDa, CD4-associated polypeptide as phosphotyrosine, and to identify this kinase as one of the members of the arc family of intracellular tyrosine kinases expressed in myeloid cells (fgr, fyn, lyn, hck). In fact, we obtained evidence that the 56 kDa phosphorylated protein is not lck nor src. The kinase activity associated with CD4 in U937 and ML3 cells correlated with CD4 expression. In fact, down-modulation of CD4 on U937 cells induced by phorbol esters was accompanied by a decreased detectability of the kinase activity in anti-CD4 immunoprecipitates. Furthermore, we observed that differentiation of ML3 cells to monocytes induced by tumor necrosis factor-alfa (TNF-a) and interferon gamma (IFN-y) was accompanied by an enhanced expression of CD4 and a parallel increase of the kinase activity in anti-CD4 immunoprecipitates. Also treatment of MDM with cytokines (IFN-y, GM-CSF) caused an enhancement of the kinase activity detected in anti-CD4 immunoprecipitates; this phenomenon did not depend on an enhanced expression of surface CD4. The observation that cytokines which induce differentiation of ML3 cells also induce surface expression of CD4, prompted studies to reveal the molecular basis of this phenomenon. We obtained evidence that TNF-a and IFN-y induce the CD4 nRNA in ML3 cells.

CONCLUSIONS: The association of a kinase activity with CD4 in myeloid cells indicates that, also in these cells, interaction of CD4 with appropriate ligands can generate a signal possibly implicated in stimulation of selective functions. Furthermore, the evidence that cytokines can modulate CD4 expression and CD4-kinase association in myeloid cells suggests that the infection of macrophages with HIV in sites of inflammation and infection can be variable.

Copyright © 1991 - International AIDS Society (IAS). Reproduction of this abstract (other than one copy for personal reference) must be cleared through the IAS.