HIV-1 Cell to Cell Transfer - Essay Example

The aim of this study is to investigate the role played by the viral surface envelope glycoprotein and the receptors within the host cells, in the direct cell to cell transfer of HIV-1. The authors have targeted their investigation on the viral surface envelope glycoprotein (Env), gp120 and the role played by its interaction with a cellular expression of CD4 and chemokine receptors (CKR), and the possible interaction of elements of the actin cytoskeleton within the host cell. They established a system to investigate the membrane dynamics of the events occurring at the interface of HIV-1 infected and receptor-expressing T cells. The authors selected appropriate CD4+/CXCR4+ T cell lines for the study maintained them in an antibiotic supplemented cell growth medium and established the purity of the cell lines at a level greater than 90% by flow cytometry by indirect immunofluorescence. These CD4+ cells were labeled as target cells. Jurkat CE 6.1 cells infected with HIV-1 strain LAI were used as effector cells. After phenotyping the cells for surface Env and CD4 expression, the effector and the target cells were mixed in equal quantities on coverslips, with or without the inclusion of mAb (monoclonal antibody).  For specific time intervals after which they were fixed and stained. Kinetic studies were conducted in separate experiments by immunostaining of conjugates for specific mAbs. Appropriate software and methods for confocal microscopy and photography were employed. Inhibition of cytoskeletal rearrangement and signaling were studied in separate experiments. Cell-cell fusion assay and transmission electron microscopy were the other experiments conducted.

The authors have been able to develop a novel system to study the cell to cell dissemination of HIV-1 by demonstrating a close packing and concentration of the virus particles in the plasma membranes of both effector and target cells, though they could not actually demonstrate the formation of a synapse between the two cells, which they suggest is the most likely mechanism. The role of an actin-dependent mechanism in the Env-dependent recruitment of CD4, CXCR4, and LFA-1 has successfully been demonstrated. This cytoskeleton-dependent receptor movement during infection of the target cells along with the formation of an adhesive junction has been proposed as the likely mechanism for the rapid cell to cell transmission of the HIV-1 virus. The role played by CD4 and CXCR4 in driving Env-Gag recruitment to the cellular interface has been demonstrated with experiments in which appropriate inhibitors were able to reduce actin-mediated receptor polarizations.

The Strengths of the Paper

The authors have used a novel approach in this comprehensive study involving a multitude of experiments to elucidate the role of viral surface glycoprotein and CD4 and chemokine receptors and encouraged the scope for further studies in identifying the role played by actin-dependent mechanisms to facilitate viral transfer between cells, which can be targeted for developing therapeutic strategies. The pathogenesis of HIV disease is under intensive research and therapeutic strategies at the current level are aimed at targeting the life cycle of the HIV virus inside the host (Havlir & Richman, 1997). Primary infection with HIV is followed by widespread dissemination throughout the lymphatic tissues. Elucidation of the mechanisms involved can thus help in furthering the cause of the therapeutic approach. Studies in the past have shown that dendritic cells, which are bone marrow-derived leukocytes, are responsible for primary and secondary immune responses to HIV infection (Zoetewij & Blauvelt, 1998). These are present in the form of Langerhans cells in the epidermis and genital mucosa and they express only the HIV co-receptor CCR5 and not CXCR4 on their cell surfaces. This would suggest a macrophage-tropic HIV transmission across the intact epithelium. Such mechanisms assist the virus in breaching mucosal surfaces after which the cell to cell transmission is initiated. The study under review tries to establish a mode for further elucidation of the mechanisms involved in virus dissemination inside the host cells. The authors have successfully demonstrated the rapid mobilization of CD4, CXCR4, talin, and lymphocyte function-associated antigen-1on the target cell, and Env and Gag on the effector cell, at the cellular interface. Remodeling of the actin cytoskeleton was discovered to be an important event. Recruitment of HIV receptors and adhesion molecules to the interface has been successfully demonstrated.

Weakness of the Study and suggestions for improvement

This particular study is quite thorough in design as well analysis and appropriate immunological staining techniques have shown the very clear cut concentration of identified markers at the cell junctions, but the actual demonstration of the hypothesized ‘synapse’ could not be done. Variations in kinetic studies and failure to detect syncytium formation and cell to cell fusion was there, which was explained by the authors on the basis of the nonuse of activated or immortalized T cells. This should not have been overlooked at the outset and can be an indicator for further studies using immortalized T cell cultures. However the study is without any major flaw as the field of HIV research is still vast and unexplored, and this study provides a stimulus in the right direction.

Further Studies, Unanswered Questions

There is the rapid spread of the HIV virus in tissues such as lymph nodes which are the primary area for virus localization after infection. Lymphoid tissue being specifically rich in CD4+ T cells, the underlying mechanisms of virus spread needs elaboration and this study has provided important clues to the involvement of receptor proteins and the role played by the actin cytoskeleton. Whether this mechanism is accurate and there is an actual synapse formation at the effector and target cellular interface, needs further studies for the elucidation of the exact mechanisms.