November 11, 2015
In an example of publication kismet, three recent open access articles all converge in describing a new strategy for depleting the HIV reservoir. The research involves engineering antibodies or antibody-like molecules capable of simultaneously binding two targets: the CD3 receptor, a human protein expressed on T cells, and parts of the HIV envelope (Env) protein, which are typically displayed on the outside of infected CD4 T cells when the virus is active. The rationale is that the region of the antibody that targets HIV Env binds to infected cells, while the CD3-targeting region binds to passing T cells and activates them to kill the cell. The idea for this two-pronged -- termed bispecific -- antibody attack originated in cancer research, and one candidate that targets CD19 (a protein expressed by B cells) and CD3 is already FDA-approved as a second-line treatment for certain forms of acute lymphoblastic leukemia.
In the Journal of Clinical Investigation, Julia Sung and colleagues report results obtained in vitro with "Dual-Affinity Re-Targeting" (DART) proteins designed to bind CD3 and HIV Env epitopes targeted by the broadly binding (but non-neutralizing) monoclonal antibodies A32 or 7B2. These DART proteins were able to significantly reduce HIV levels in cultured, HIV-infected CD4 T cells and also showed activity against latently infected CD4 T cells isolated from individuals on ART and exposed to the latency-reversing agent vorinostat.
In PLoS Pathogens, a research team from Gilead Sciences and MacroGenics, Inc. describe similar studies conducted with a larger array of candidate DART proteins targeting CD3 and one of several different HIV Env epitopes, including those bound by A32, 7B2 and the broadly neutralizing antibodies PGT121, PGT145, VRC01 and 10E8. The most active DART proteins were those derived from PGT121, PGT145, A32 and 7B2, confirming and extending the results of Sung et al. Of potential importance for clinical development, modified versions of the DART proteins designed to allow for relatively infrequent dosing in humans maintained activity.
An important, overarching concern with all these bispecific antibodies is the extent to which the CD3-binding region might cause generalized T cell activation. Many years ago, anti-CD3 antibodies were tested in people with HIV with the aim of broadly activating CD4 T cells as means to reverse latency and deplete the HIV reservoir, but the results were disastrous: the treatment led to a massive depletion of T cells, inflammatory cytokine release (leading to transient renal failure in one case) and no evident HIV reservoir reduction (see Prins et al,1999).
All three research groups conducted laboratory assessments of T cell activation, and found little or no evidence that it occurred in the absence of HIV Env expression; in other words, binding of the bispecific antibodies to both targets appeared necessary for inducing activation. However, Pegu et al also performed a safety assessment in SHIV-infected macaques receiving antiretroviral therapy and this revealed a sharp but short-lived drop in peripheral blood CD3+ T cells and a sizable, albeit transient, increase in levels of several inflammation-related cytokines (TNF-α, MIP-1β and IL-10). These immunological perturbations resolved within 24 hours and there were no increases in SHIV viral load or clinically evident adverse events, leading the researchers to conclude that: "overall, these short-term toxicity studies indicate that the treatment was well tolerated."
Taken together, the three papers suggest that bispecific antibodies may have promise as an anti-reservoir strategy. A particular appeal of the approach is the capacity to recruit T cells to destroy HIV-infected cells without regard to their antigen specificity -- this could be important because HIV-specific T cells are typically functionally compromised in HIV-positive people. Tempering enthusiasm somewhat are the macaque data showing transient T cell depletion and inflammation, a phenomenon that will need to be evaluated in further animal studies. Barring additional safety concerns arising, human trials appear likely given the involvement of Gilead Sciences (a company with a large HIV cure research program).
Richard Jefferys is the coordinator of the Michael Palm HIV Basic Science, Vaccines & Prevention Project Weblog at the Treatment Action Group (TAG). The original blog post may be viewed here.
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