The innate immune system and HIV pathogenesis

Regulatory role of natural killer (NK) cells on antibody responses to Brucella abortus

Our previous studies have indicated an important regulatory role for natural killer (NK) cells, a major constituent of the innate immune system in modulating antigen-specific responses. Herein, we have investigated the possible participation of these cells in regulating the polyclonal response as well. For these studies we have utilized heat-killed Brucella abortus (HKBA). Brucella abortus is a facultative intracellular bacterium that is pathogenic for both humans and animals. An outstanding feature of the infectious process is the rapid production of polyclonal antibodies, particularly of the IgG2c subclass, that bypasses the requirement for clonally specific antigen recognition. We report here that NK-cell depletion profoundly reduced the production of these polyclonal antibodies suggesting that activation of B cells by HKBA requires help from NK cells. This help may not be solely derived from NK-cell amplification of the cytokine circuit initiated by HKBA but may involve direct NK-B-cell interactions as suggested by results of in vitro analyses of NK induction of γ2a mRNA by B cells. These findings have therapeutic implications in that the induction of polyclonal Ig production may be more important for altering the chronic phase rather than the acute stage of infection by B. abortus.

HIV-1 gp120-induced migration of dendritic cells is regulated by a novel kinase cascade involving Pyk2, p38 MAP kinase, and LSP1

Targeting dendritic cell (DC) functions such as migration is a pivotal mechanism used by HIV-1 to disseminate within the host. The HIV-1 envelope protein is the most important of the virally encoded proteins that exploits the migratory capacity of DCs. In the present study, we elucidated the signaling machinery involved in migration of immature DCs (iDCs) in response to HIV-1 envelope protein. We observed that M-tropic HIV-1 glycoprotein 120 (gp120) induces phosphorylation of the nonreceptor tyrosine kinase, Pyk2. Inhibition of Pyk2 activity using a pharmacologic inhibitor, kinase-inactive Pyk2 mutant, and Pyk2-specific small interfering RNA blocked gp120-induced chemotaxis, confirming the role of Pyk2 in iDC migration. In addition, we also illustrated the importance of Pyk2 in iDC migration induced by virion-associated envelope protein, using aldithriol-2-inactivated M-tropic HIV-1 virus. Further analysis of the downstream signaling mechanisms involved in gp120-induced migration revealed that Pyk2 activates p38 mitogen-activated protein kinase, which in turn activates the F-actin-binding protein, leukocyte-specific protein 1, and enhances its association with actin. Taken together, our studies provide an insight into a novel gp120-mediated pathway that regulates DC chemotaxis and contributes to the dissemination of HIV-1 within an infected person.

Use of an anti-CD16 antibody for in vivo depletion of natural killer cells in rhesus macaques

Non-human primates serve as key animal models for a variety of viral infections. To evaluate the contribution of natural killer (NK) cells to the immune-mediated control of these viruses in macaque monkeys, we have described a method for depleting NK cells in vivo by administration of anti-human CD16 mouse monoclonal antibody. Using a fluorometric NK-cell cytotoxicity assay, we show that most NK-cell cytotoxicity in rhesus monkey peripheral blood mononuclear cells resides in the CD16(+) and/or CD159A(+) subset of lymphocytes. The anti-human CD16 antibody, 3G8, binds to subsets of rhesus monkey lymphocytes and monocytes but not to neutrophils. Intravenous administration of 10-50 mg/kg of 3G8 to normal rhesus monkeys resulted in anti-CD16 antibody persistence in the plasma for 1-3 weeks. This treatment also depleted 80-90% of CD3(-) CD159A(+) lymphocytes, putative NK cells, from blood for at least 1 week and was associated with the loss of NK-cell cytotoxicity when evaluated by in vitro assays. Using this method, transient depletion of NK cells from two rhesus monkeys chronically infected with simian immunodeficiency virus failed to cause changes in virus replication. These studies describe a non-human primate model for in vivo NK-cell depletion and suggest a limited role for cytotoxic CD16(+) NK cells in controlling AIDS virus replication during chronic infection.

A conserved molecular action of native and recombinant Epap-1 in inhibition of HIV-1 gp120 mediated viral entry

The early expression of Epap-1 (early pregnancy associated protein), a 90 kDa anti-HIV-1 active glycoprotein, in the first trimester placental tissue suggests that it is one of the innate immune factors/proteins protecting the fetus from HIV infections. In the present investigation, we have cloned and expressed Epap-1 in bacterial and baculovirus expression systems. The recombinant Epap-1 as well as native Epap-1 shows a conserved molecular mode of action. These proteins exhibit significant antiviral activity and inhibit the cell fusion reaction between gp120 expressing HeLa (HL2/3) cells and T cell line (SupT1). Further, the rhodamine labeled Epap-1 specifically bound to gp120 expressed on the surface of HL2/3 cells during fusion reaction thereby inhibiting viral entry. Analysis of the interacting gp120 epitopes revealed that Epap-1 binds specifically to epitopes of gp120, recognizing constant-5 (C5) region and the variable-3 (V3) epitope of gp120 expressed on HL2/3 cells; It exhibits specific interaction with C5 region of cell-free virus in four HIV-1 isolates suggesting that the molecular interaction of Epap-1 is specific and is highly conserved in binding to gp120 leading to inhibition of viral entry. Epap-1 can thus be a very efficient natural protection mechanism against cell-free and cell-associated viral infections during early pregnancy.

The antigenic determinants on HIV p24 for CD4+ T cell inhibiting antibodies as determined by limited proteolysis, chemical modification, and mass spectrometry

In the last decade, mass spectrometry has been employed by more and more researchers for identifying the proteins in a macromolecular complex as well as for defining the surfaces of their binding interfaces. This characterization of protein-protein interfaces usually involves at least one of several different methodologies in addition to the actual mass spectrometry. For example, limited proteolysis is often used as a first step in defining regions of a protein that are protected from proteolysis when the protein of interest is part of a macromolecular complex. Other techniques used in conjunction with mass spectrometry for determining regions of a protein involved in protein-protein interactions include chemical modification, such as covalent cross-linking, acetylation of lysines, hydrogen-deuterium exchange, or other forms of modification. In this report, both limited proteolysis and chemical modification were combined with several mass spectrometric techniques in efforts to define the protein surface on the HIV core protein, p24, recognized by two different monoclonal human antibodies that were isolated from HIV+ patients. One of these antibodies, 1571, strongly inhibits the CD4+ T cell proliferative response to a known epitope (PEVIPMFSALSEGATP), while the other antibody, 241-D, does not inhibit as strongly. The epitopes for both of these antibodies were determined to be discontinuous and localized to the N-terminus of p24. Interestingly, the epitope recognized by the strongly inhibiting antibody, 1571, completely overlaps the T cell epitope PEVIPMFSALSEGATP, while the antibody 241-D binds to a region adjacent to the region of p24 recognized by the antibody 1571. These results suggest that, possibly due to epitope competition, antibodies produced during HIV infection can negatively affect CD4+ T cell-mediated immunity against the virus.