Sialoadhesin Modulates HIV-1 Infectivity

D. Lynn Pulliam, Northern Californian Institute for Research and Education

The central nervous system (CNS) constitutes an important viral reservoir created by HIV-1-infected monocyte/macrophages (M/Mø) transmigrating from the peripheral blood into the perivascular region. This reservoir is especially problematic because infected and activated M/Mø secrete inflammatory and neurotoxic factors causing neurodegeneration, culminating in cognitive impairment. Since it is widely accepted that high viral load (HVL) is a risk factor for AIDS and HIV-1-associated dementia (HAD), we examined the phenotype of monocytes from at-risk subjects. We performed global gene expression analysis of circulating CD14+ monocytes isolated from HIV-1-infected individuals with HVL ( > 10,000 RNA copies/ml). Of the many differentially expressed genes, we selected sialoadhesin (Sn; CD169) for further analysis because of its role in cell – cell adhesion. Compared to controls, Sn exhibited a 19-fold increase (p < 0.05) in subjects with HVL. Sialoadhesin is a large, extracellular protein with an N-terminal domain which binds sialic acid and was initially characterized as a macrophage-restricted protein. Flow cytometry and immunocytochemical analyses of CD14+ monocytes isolated from HIV-1-infected individuals with HVL confirmed the elevated Sn expression found in the array data. To examine the potential biological relevance of Sn expression, THP-1 cells were transfected with a CMV-SN contruct. In vitro, Sn-expressing THP-1 cells demonstrated a capacity to bind replication deficient HIV-1 (HIV- 1∆Tat/Rev) in a robust and highly specific manner.

The overall objective of this pilot study is to determine the biological relevance of our novel in vitro observations that Sn is elevated on monocytes from subjects with HVL and that Sn avidly binds HIV-1 via terminal sialic acids on the virion. Our first specific aim is to determine if HIV-1 is bound to Sn in vivo. Preliminary data clearly show that HIV-1 binds Sn in vitro and that Sn expression is elevated in HIV-1- infected subjects with a high viral load (HVL.). Based on these observations, we will examine the in vivo interaction between Sn and HIV-1 by assaying CD14+ monocytes for Sn-bound HIV-1 from subjects with HVL. The second aim is to determine if Sn-bound HIV-1 on the surface of CD14+ monocytes could infect susceptible T cells in trans. In this aim we will explore the functionality for Sn; can it effectively present HIV-1 to susceptible T cells? Results from these experiments may identify a new mechanism for HIV-1 attachment to Sn-expressing monocytes as well as prove that Sn-expressing monocytes facilitate infection of susceptible T cells. This mechanism may be a significant new therapeutic target.