Background: Interactions between the HIV-1 envelope glycoprotein (Env) and its primary receptor CD4 are
influenced by the physiological setting in which these events take place. In this study, we explored the surface
chemistry of HIV-1 Env constructs at a range of pH and salinities relevant to mucosal and systemic compartments
through electrophoretic mobility (EM) measurements. Sexual transmission events provide a more acidic
environment for HIV-1 compared to dissemination and spread of infection occurring in blood or lymph node. We
hypothesize functional, trimeric Env behaves differently than monomeric forms.
Results: The dynamic electrophoretic fingerprint of trimeric gp140 revealed a change in EM from strongly negative
to strongly positive as pH increased from that of the lower female genital tract (pHx) to that of the blood (pHy).
Similar findings were observed using a trimeric influenza Haemagglutinin (HA) glycoprotein, indicating that this
may be a general attribute of trimeric viral envelope glycoproteins. These findings were supported by
computationally modeling the surface charge of various gp120 and HA crystal structures. To identify the behavior
of the infectious agent and its target cells, EM measurements were made on purified whole HIV-1 virions and
primary T-lymphocytes. Viral particles had a largely negative surface charge, and lacked the regions of positivity
near neutral pH that were observed with trimeric Env. T cells changed their surface chemistry as a function of
activation state, becoming more negative over a wider range of pH after activation. Soluble recombinant CD4
(sCD4) was found to be positively charged under a wide range of conditions. Binding studies between sCD4 and
gp140 show that the affinity of CD4-gp140 interactions depends on pH.
Conclusions: Taken together, these findings allow a more complete model of the electrochemical forces involved
in HIV-1 Env functionality. These results indicate that the influence of the localized environment on the interactions
of HIV with target cells are more pronounced than previously appreciated. There is differential chemistry of trimeric,
but not monomeric, Env under conditions which mimic the mucosa compared to those found systemically. This
should be taken into consideration during design of immunogens which targets virus at mucosal portals of entry.