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Physiological Levels of Virion-Associated Human Immunodeficiency Virus Type 1 Envelope Induce Coreceptor-Dependent Calcium Flux ^,

Department of Cell and Molecular Biology, Northwestern University, Chicago, Illinois 60611,¹ Department of Microbiology and Immunology, University of Illinois at Chicago, Chicago, Illinois 60612,² Retroviral Assembly Laboratory, SAIC-Frederick, NCI at Frederick, Frederick, Maryland 21702³

Received 22 June 2006/ Accepted 15 November 2006

Human immunodeficiency virus (HIV) entry into target cells requires the engagement of receptor and coreceptor by envelope glycoprotein (Env). Coreceptors CCR5 and CXCR4 are chemokine receptors that generate signals manifested as calcium fluxes in response to binding of the appropriate ligand. It has previously been shown that engagement of the coreceptors by HIV Env can also generate Ca²⁺ fluxing. Since the sensitivity and therefore the physiological consequence of signaling activation in target cells is not well understood, we addressed it by using a microscopy-based approach to measure Ca²⁺ levels in individual CD4⁺ T cells in response to low Env concentrations. Monomeric Env subunit gp120 and virion-bound Env were able to activate a signaling cascade that is qualitatively different from the one induced by chemokines. Env-mediated Ca²⁺ fluxing was coreceptor mediated, coreceptor specific, and CD4 dependent. Comparison of the observed virion-mediated Ca²⁺ fluxing with the exact number of viral particles revealed that the viral threshold necessary for coreceptor activation of signaling in CD4⁺ T cells was quite low, as few as two virions. These results indicate that the physiological levels of virion binding can activate signaling in CD4⁺ T cells in vivo and therefore might contribute to HIV-induced pathogenesis.

Published ahead of print on 22 November 2006.

Supplemental material for this article may be found at http://jvi.asm.org/.

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