Division of Basic Science

Alfredo Garzino-Demo, Ph.D.
Research Assistant Professor,
University of Maryland Biotechnology Institute
agarzinod@ihv.umaryland.edu

Discoveries over the past few years have defined a close relationship between HIV-1 infection and the portion of the immune system involving chemokines. Our discovery that the b chemokines RANTES, MIP-1a and MIP-1b suppress HIV-1 provided the initial connection and indicated that these molecules might control infection as part of immune responses in vivo (1). Several groups have identified chemokine receptors or chemokine receptor-like molecules as co-receptors for HIV(reviewed in ref. 2). All of these co-receptors are part of the large and functionally diverse family of the seven-transmembrane G-protein-coupled receptors. The first identified co-receptor, CXCR4, is a receptor for the a (CXC) subclass of chemokines and mediates the entry/fusion of T-cell tropic strains of HIV. Another receptor for the members of the b (CC) chemokine subclass, CCR-5, mediates entry and fusion of macrophage-tropic isolates of HIV. This molecule serves as a receptor for RANTES, MIP-1a and MIP-1b and thus provides the basis for our original observations that these CCR5 ligands suppress HIV(1).

Suppression of HIV by chemokines represents a special case in virology and immunology where soluble molecules other than antibodies inhibit infection by a specific virus. Consequently, our studies have focused on the role of chemokines that bind to CCR5 in protection from infection and disease progression. In particular, we are interested in studying the release of CCR5 ligands in response to antigen activation. Our study (Fig. 1) with the Multicenter AIDS Cohort Study (MACS) showed that AIDS-free status and higher CD4+T cell counts correlate with higher CCR5 ligands release from PBMC after activation in vitro by HIV antigen (3). In addition, we found that cells from individuals who remain seronegative despite high-risk exposure to HIV infection release significantly higher levels of chemokines than normal seronegative controls and HIV-positive subjects, even in absence of an in vitro stimulus (3).

We are now studying the mechanistic aspect of CCR5 ligands release in response to antigen activation, using the prototypical Tetanus toxoid antigen (Fig 2). Our results indicate that recall antigen stimulation can be easily measured by chemokine release at time points where little or no T cell proliferation is observed, and suggests that chemokine release is a very early event in the consequence of stimulation by recall antigens. Thus, it is likely that chemokine secretion can be readily measured in HIV immunization protocols, and we intend to pursue the study of antigen-activated chemokines in such protocols as potential correlates of protection, alone or in combination with other immune response parameters. In addition, we are investigating how CCR5 ligands expression is regulated at the molecular level.

Another recently discovered chemokine, MDC, a ligand for CCR4, appears to represent a class of chemokines that suppress HIV infection in a novel manner that does not directly involve blocking the CCR5 or CXCR4 coreceptors (4). Cell-free infections and pseudotype assays have shown dose-dependent suppression of both R5 and X4 strains. Suppression of X4 strains is reversible by pertussis toxin and is therefore Gi-protein dependent, whereas suppression of R5 strains is not reversible by pertussis toxin. Thus, our results suggest that suppression of viral replication by MDC occurs through different mechanisms according to co-receptor tropism, and the characterization of the antiviral mechanisms induced by this molecule is vigorously pursued in our laboratory.