Background:  CD4 plays a central role in immune function and as the primary receptor for HIV-1.  Infection is initiated by binding of gp120 to CD4 and a G-protein coupled co-receptor, either CCR5 (R5 strains) or CXCR4 (X4 strains).  CD4 lacks intrinsic kinase activity.  In lymphocytes, engagement of CD4 leads to activation of the src kinase p56^lck, resulting in its dissociation from CD4 and the activation of signaling pathways.  Furthermore, binding of HIV-1 or recombinant gp120 to CD4 in lymphocytes results in the elevation of intracellular calcium and activation of several protein tyrosine kinases, including members of the mitogen-activated protein kinase (MAP) family.  In contrast, primary human monocyte-derived macrophages (MDM) do not express p56^lck and the intracellular signals elicited through CD4 on MDM in response to HIV-1 gp120 remain largely unknown. 

Methods:  Toward this end, we examined the signaling responses evoked in MDM upon engagement of CD4, CCR5, or both.  Our results demonstrate that engagement of CD4 with specific antibody results in rapid and transient phosphorylation of the MAP kinase ERK-1/2. 

Results:  Stimulation of MDM with the CCR5 agonist MIP-1 or with recombinant gp120 from the R5 strain JRFL, also lead to phosphorylation of the MAP kinase ERK-1/2 and, additionally, phosphorylation of the MAP kinase p38.  Specific inhibitors of CCR5 blocked phosphorylation of ERK-1/2 and p38 following MIP-1  stimulation.  Following JRFL gp120 stimulation, however, CCR5 inhibitors blocked activation of p38 only, but did not prevent ERK-1/2 phosphorylation.  Thus, CD4 in MDMs is linked to ERK-1/2, while CCR5 is linked to both ERK-1/2 and p38.  Furthermore, R5 HIV-1 gp120 utilizes both CD4 and CCR5 signaling pathways. 

Conclusions:  Our findings may provide insight into the initial signaling events that occur upon HIV-1 binding in macrophages that may contribute to macrophage dysfunction in AIDS.

Keywords: CD4; macrophages; MAP kinase