Macrophage-Derived Factors with the Potential to Contribute to Pathogenicity of HIV-1 and HIV-2: Role of CCL-2/MCP-1

Human immunodeficiency virus type 2 (HIV-2) is known to be less pathogenic than HIV-1. However, the mechanism(s) underlying the decreased HIV-2 pathogenicity is not fully understood. Herein, we report that β-chemokine CCL2 expression was increased in HIV-1-infected human monocyte-derived macrophages (MDM) but decreased in HIV-2-infected MDM when compared to uninfected MDM. Inhibition of CCL2 expression following HIV-2 infection occurred at both protein and mRNA levels. By microarray analysis, quantitative PCR, and Western blotting, we identified that Signal Transducer and Activator of Transcription 1 (STAT1), a critical transcription factor for inducing CCL2 gene expression, was also reduced in HIV-2-infected MDM. Blockade of STAT1 in HIV-infected MDM using a STAT1 inhibitor significantly reduced the production of CCL2. In contrast, transduction of STAT1-expressing pseudo-retrovirus restored CCL2 production in HIV-2-infected MDM. These findings support the concept that CCL2 inhibition in HIV-2-infected MDM is meditated by reduction of STAT1. Furthermore, we showed that STAT1 reduction in HIV-2-infected MDM was regulated by the CUL2/RBX1 ubiquitin E3 ligase complex-dependent proteasome pathway. Knockdown of CUL2 or RBX1 restored the expression of STAT1 and CCL2 in HIV-2-infected MDM. Taken together, our findings suggest that differential regulation of the STAT1-CCL2 axis may be one of the mechanisms underlying the different pathogenicity observed for HIV-1 and HIV-2.

Platelet factor 4 (CXCL4) facilitates human macrophage infection with HIV-1 and potentiates virus replication

Platelet factor 4 (CXCL4), a member of the CXC chemokine subfamily released in high amounts by activated platelets, has been identified as a monocyte survival factor that induces monocyte differentiation into macrophages. Although CXCL4 has been shown to have biological effects unique to chemokines, nothing is known about the role of CXCL4-derived human macrophages or CXCL4 in human immunodeficiency virus (HIV) disease. In this study, CXCL4-derived macrophages are compared with macrophage-colony stimulating factor (M-CSF)-derived macrophages for their ability to support HIV-1 replication. We show that CXCL4-derived macrophages can be infected with macrophage-tropic HIV-1 that uses either CC-chemokine receptor 5 (CCR5) or CXC-chemokine receptor 4 (CXCR4) as a co-receptor for viral entry. We also find that M-CSF and the chemokines, monocyte chemoattractant protein 1 (MCP-1; CCL2) and macrophage-inflammatory-protein-1-alpha (MIP-1alpha; CCL3) are produced upon R5- and X4-tropic HIV-1 replication in both M-CSF- and CXCL4-derived human macrophages. In addition, CXCL4 added to M-CSF-derived macrophages after virus adsorption and maintained throughout the infection enhances HIV-1 replication. We thus propose a novel role for CXCL4 in HIV disease.

Simian immunodeficiency viruses from multiple lineages infect human macrophages: implications for cross-species transmission

Zoonotic transfer of simian immunodeficiency virus (SIV) from chimpanzees and sooty mangabeys to humans has been documented on at least seven occasions. Several recently identified SIV isolates have also been shown to replicate efficiently in human peripheral blood mononuclear cells (PBMCs) in vitro, indicative of the potential for additional cross-species transmission via T cell infection. Although SIV predominantly uses the macrophage-tropic HIV chemokine coreceptor CCR5, little is known about the ability of SIV to infect human macrophages. In this study, 16 SIV isolates belonging to five different primate lentivirus lineages were tested for their ability to infect human monocyte-derived macrophages (MDMs). Twelve of the viruses were capable of infecting MDMs, and 11 of these were also able to replicate in human PBMCs. The replication capacity of the isolates differed within and between the various families and was dependent on particular donor macrophages. Our results suggest that most simian lentiviruses characterized to date not only have the ability to infect primary human T lymphocytes but also replicate efficiently in macrophages, thereby increasing the potential for cross-species transmission into the human population. Comparative studies using these isolates may facilitate the identification of characteristics that contribute to virus infectivity and pathogenicity.

Rhesus monkey simian immunodeficiency virus infection as a model for assessing the role of selenium in AIDS

The objective of this study was to determine whether simian immunodeficiency virus (SIV) infection of macaques could be used as a model system to assess the role of selenium in AIDS. Plasma and serum selenium levels were determined by standard assays in monkeys before and after inoculation of SIV. SIV-infected cells or cells expressing the HIV Tat protein were labeled with 75Se, and protein extracts were prepared and electrophoresed to analyze selenoprotein expression. Total tRNA was isolated from CEMx174 cells infected with SIV or from KK1 cells infected with HIV, and selenocysteine tRNA isoforms were characterized by reverse phase chromatography. SIV-infected monkeys show a decrease in blood selenium levels similar to that observed in AIDS with development of SAIDS. Cells infected with SIV in vitro exhibit reduced selenoprotein levels and an accumulation of small molecular weight selenium compounds relative to uninfected cells. Examination of the selenocysteine tRNA isoforms in HIV-infected KK1 cells or SIV-infected CEMx174 cells reveals an isoform distribution characteristic of selenium-deficient cells. Furthermore, transfection of Jurkat E6 cells with the Tat gene selectively altered selenoprotein synthesis, with GPX4 and Sep15 being the most inhibited and TR1 the most enhanced. Taken together, the data show that monkeys infected with SIV in vivo and cells infected with SIV in vitro will provide appropriate models for investigating the mechanism(s) responsible for reduced selenium levels that accompany the progression of AIDS in HIV disease.

Inhibition of HIV replication and macrophage colony-stimulating factor production in human macrophages by antiretroviral agents

Macrophage colony-stimulating factor (M-CSF) enhances the susceptibility of macrophages to infection with HIV-1, in part by increasing the expression of CD4 and CCR5. Human monocyte-derived macrophages (MDMs) infected in vitro with HIV-1 endogenously produce M-CSF, with kinetics paralleling virus replication, which can lead to enhanced spreading of the infection. AZT and ritonavir both inhibit HIV replication, but their impact on M-CSF production by HIV-infected human MDMs is unknown. The dose response and kinetics of virus replication in the presence of AZT and ritonavir were determined for HIV-infected MDMs from HIV-seronegative donors. Harvested supernatants were monitored for reverse transcriptase activity, M-CSF production, and HIV proteins. Our data suggest that threshold levels of HIV replication must occur before maximum M-CSF production is induced. Addition of AZT or ritonavir before or after establishment of productive HIV infection dramatically reduces virus replication and M-CSF production by human MDMs. However, ongoing virus replication and M-CSF production are slow to return to baseline levels after addition of AZT or ritonavir, suggesting that HIV replication and virion release from infected macrophages continue long after initiation of antiretroviral therapy. Our results suggest that, in human macrophages, HIV-1 replication and M-CSF production are inextricably linked, such that inhibition of one leads to a concomitant reduction of the other. Low-level HIV replication and M-CSF release during ongoing antiretroviral therapies may facilitate the survival and maintenance of infected macrophages and suggests that additional therapies targeting M-CSF may be critical for elimination of macrophage reservoirs.

Down-regulation of neutrophil functions by the ELR(+) CXC chemokine platelet basic protein

The platelet-derived neutrophil-activating peptide 2 (NAP-2, 70 amino acids) belongs to the ELR(+) CXC subfamily of chemokines. Similar to other members of this group, such as IL-8, NAP-2 activates chemotaxis and degranulation in neutrophils (polymorphonuclear [PMN]) through chemokine receptors CXCR-1 and CXCR-2. However, platelets do not secrete NAP-2 as an active chemokine but as the C-terminal part of several precursors that lack PMN-stimulating capacity. As we have previously shown, PMN themselves may liberate NAP-2 from the precursor connective tissue-activating peptide III (CTAP-III, 85 amino acids) by proteolysis. Instead of inducing cell activation, continuous accumulation of the chemokine in the surroundings of the processing cells results in the down-regulation of specific surface-expressed NAP-2 binding sites and in the desensitization of chemokine-induced PMN degranulation. Thus, NAP-2 precursors may be regarded as indirect mediators of functional desensitization in neutrophils. In the current study we investigated the biologic impact of another major NAP-2 precursor, the platelet basic protein (PBP, 94 amino acids). We show that PBP is considerably more potent than CTAP-III to desensitize degranulation and chemotaxis in neutrophils. We present data suggesting that the high desensitizing capacity of PBP is based on its enhanced proteolytic cleavage into NAP-2 by neutrophil-expressed cathepsin G and that it involves efficient down-regulation of surface-expressed CXCR-2 while CXCR-1 is hardly affected. Correspondingly, we found PBP and, less potently, CTAP-III to inhibit CXCR-2- but not CXCR-1- dependent chemotaxis of neutrophils toward NAP-2. Altogether our findings demonstrate that the anti-inflammatory capacity of NAP-2 is governed by the species of its precursors.