Monocyte maturation, HIV susceptibility, and transmigration across the blood brain barrier are critical in HIV neuropathogenesis

Abstract

HIV continues to be a global health crisis with more than 34 million people infected worldwide (UNAIDS: Report on the Global AIDS Epidemic 2010, Geneva, World Health Organization). HIV enters the CNS within 2 weeks of infection and establishes a spectrum of HAND in a large percentage of infected individuals. These neurologic deficits greatly impact the quality of life of those infected with HIV. The establishment of HAND is largely attributed to monocyte transmigration, particularly that of a mature CD14(+)CD16(+) monocyte population, which is more susceptible to HIV infection, across the BBB into the CNS parenchyma in response to chemotactic signals. To enter the CNS, junctional proteins on the monocytes must participate in homo- and heterotypic interactions with those present on BMVECs of the BBB as they transmigrate across the barrier. This transmigration is responsible for bringing virus into the brain and establishing chronic neuroinflammation. While there is baseline trafficking of monocytes into the CNS, the increased chemotactic signals present during HIV infection of the brain promote exuberant monocyte transmigration into the CNS. This review will discuss the mechanisms of monocyte differentiation/maturation, HIV infectivity, and transmigration into the CNS parenchyma that contribute to the establishment of cognitive impairment in HIV-infected individuals. It will focus on markers of monocyte subpopulations, how differentiation/maturation alters HIV infectivity, and the mechanisms that promote their increased transmigration across the BBB into the CNS.

Figure 1.. CD14⁺CD16⁺ monocytes increase in numbers in nonadherent culture conditions over 3 days.

Human monocytes were isolated by CD14-positive selection from PBMCs (two representative individuals are shown, Donor 1 and 2). (A) Freshly isolated and (B) monocytes maturing for 3 days in nonadherent conditions with M-CSF in our culture system were stained with CD14 APC and CD16 PECy7-coupled antibodies. CD14⁺CD16⁺ monocytes are present in low numbers in freshly isolated, circulating monocytes. Their numbers significantly increase after 3 days in nonadherent culture conditions. This increase varies among donors.

Figure 2.. Schematic representation of the correlation of monocyte maturation in vivo with our nonadherent culture system of CD14⁺ monocytes.

Circulating peripheral blood monocytes are mainly CD14⁺, and a small population also expresses CD16. In the blood, these CD14⁺CD16⁺ cells represent 5–10% of the monocyte population. In our nonadherent culture system, these cells greatly increase in numbers (see text and Fig. 1). The CD14⁺CD16⁺ cells in the blood and in our model culture system also express CD11b, CD163, PrP^C, Mac387, CD166, CCR2, and CX₃CR1. This CD14⁺CD16⁺ maturing monocyte subset is susceptible to HIV infection and preferentially transmigrates across the BBB in response to specific chemokine gradients. Upon transmigration across the BBB, these cells differentiate into monocyte/macrophages.

Figure 3.. Both CCR2 and CX₃CR1 are present on freshly isolated and mature monocytes.

(A) Freshly isolated monocytes or (B) mature monocytes cultured nonadherently with M-CSF for 3 days in our system were stained with CD14 PE and CD16 PECy7 and CCR2 AlexaFluor 647 or CX₃CR1 FITC-coupled antibodies and analyzed by flow cytometry. Histograms of CCR2 and CX₃CR1 include the mean fluorescence intensity (MFI) as compared with its IgG isotype-matched control antibody.

Figure 4.. Monocyte transmigration across the BBB.

Monocyte transmigration is a multistep process, (A) consisting of capture and rolling, chemokine-mediated activation of the monocyte and subsequent firm arrest, intravascular crawling with the formation of focal adhesions to the vasculature, and ultimately, diapedesis. (B) An enlarged image illustrating some of the homotypic monocyte–EC interactions, which mediate diapedesis.

Figure 5.. HIV-infected monocytes transmigrate exuberantly as compared to uninfected monocytes across the BBB in response to CCL2.

Maturing monocytes cultured nonadherently with M-CSF in our system, either HIV-infected or uninfected, were added to the top of our in vitro model of the BBB and transmigrated in response to media alone or 200 ng/mL CCL2 for 24 h. CCL2 resulted in significantly higher numbers of HIV-infected transmigrated CD14⁺CD16⁺ monocytes relative to baseline and to the transmigration of uninfected cells to CCL2 (*P<0.03; n=5). There was also significant transmigration of uninfected monocytes in response to CCL2 (*P<0.03; n=5). Baseline transmigration was similar for the HIV-infected and uninfected mature monocytes.