Human immunodeficiency virus-1 crosses the blood-brain barrier (BBB) to infect the central nervous system (CNS). It crosses the BBB both within infected immune cells and as free virus. How free HIV-1 crosses the BBB is unclear as the brain endothelial cells that comprise the vascular BBB are both CD4 and galactosylceramide negative. In vivo and in vitro studies show that interactions between the HIV-1 viral coat glycoprotein gp120 and unknown glycoproteins on the cell membranes of brain endothelia interact to induce a type of vesicular transport termed adsorptive transcytosis, a transport that is enhanced by protamine sulfate, wheatgerm agglutinin, and heparan sulfate, all of which likely act by binding to the unknown glycoproteins.
Evidence suggests that high mannose type glycans are involved in HIV-1 internalization. High mannose glycans are one type of carbohydrate moiety found on gp120. By way of these mannose residues, gp120 can bind to and be internalized by macrophages through a CD4-independent, mannose-specific endocytic receptor pathway; this is consistent with the high mannose sites being related to HIV-1 infectivity.
One such mannose binding receptor is the insulin-like growth factor-II/mannose-6 phosphate receptor (M6PR). This multi-ligand transporter uniquely binds phosphorylated mannose (M6P), a feature it uses to identify and route lysosomal enzymes to the lysosomal compartment. HIV-1 is also routed to lysosomes by an unknown endocytic process that is independent of CD4. Mannose-6 phosphate can inhibit gp160 binding, suggesting that gp160 binds to the M6PR; furthermore, microglial uptake of HIV-1 involves M6PR. The M6PR is involved in the trafficking of at least one other virus: herpes varicella-zoster. The M6PR has recently been shown to be an inducible transporter at the BBB for lysosomal enzymes. These various lines of evidence led us to postulate that the M6PR could be involved in the transport of HIV-1 across the BBB.
HIV-1 circulates both as free virus and within immune cells, with the level of free virus being predictive of clinical course. Both forms of HIV-1 cross the blood-brain barrier (BBB) and much progress has been made in understanding the mechanisms by which infected immune cells cross the blood-brain barrier BBB. How HIV-1 as free virus crosses the BBB is less clear as brain endothelial cells are CD4 and galactosylceramide negative. Here, we found that HIV-1 can use the mannose-6 phosphate receptor (M6PR) to cross the BBB. Brain perfusion studies showed that HIV-1 crossed the BBB of all brain regions consistent with the uniform distribution of M6PR. Ultrastructural studies showed HIV-1 crossed by a transcytotic pathway consistent with transport by M6PR. An in vitro model of the BBB was used to show that transport of HIV-1 was inhibited by mannose, mannan, and mannose-6 phosphate and that enzymatic removal of high mannose oligosaccharide residues from HIV-1 reduced transport. Wheatgerm agglutinin and protamine sulfate, substances known to greatly increase transcytosis of HIV-1 across the BBB in vivo, were shown to be active in the in vitro model and to act through a mannose-dependent mechanism. Transport was also cAMP and calcium-dependent, the latter suggesting that the cation-dependent member of the M6PR family mediates HIV-1 transport across the BBB. We conclude that M6PR is an important receptor used by HIV-1 to cross the BBB.