Cytochalasin D Modulates CD4 Crosslinking Sensitive Mitogenic Signal in T Lymphocytes

It has previously been shown that crosslinking of the CD4 molecule, either with anti-Leu3a mAb or with gp120 (the HIV coat protein) plus anti-gp120 mAb, suppresses activation induced by wt31, a TcR/CD3-specific mAb. This suppression was associated with hindrance of the necessary association of the p56lck kinase bearing CD4 molecule with the TcR/CD3 complex. In this paper we demonstrate that this crosslinking-induced suppression can be bypassed by perturbing the microfilament system of CD4+ cells by pretreatment with 1 μM cytochalasin D. Using the fluorescence resonance energy transfer method, we have shown that the cytochalasin D-affected increase of mitogenesis is not due to changes in the TcR/CD3 to CD4 distance. Likewise, other membrane biophysical parameters, membrane potential and lateral diffusion of surface receptors, cannot be associated with these cytochalasin D-affected mitogenic changes. Cytochalasin D treatment elevates intracellular Ca2+ levels induced by wt31 mAb plus crosslinking and generates a TcR/CD3-dependent signal which is cyclosporin sensitive.