Several years ago, researchers led by Jason Brenchley and Daniel Douek at the National Institute of Allergy & Infectious Diseases published data suggesting an important role for microbial translocation in HIV pathogenesis. Microbial translocation is the leaking of normally friendly commensal bacteria from the gut – where they are usually contained – into the systemic circulation. Brenchley and colleagues proposed that this phenomenon contributes to immune activation in people with HIV, and thus plays a causative role in the progression of the disease.
Several subsequent studies have confirmed an association between markers of microbial translocation found in the bloodstream (e.g. the bacterial component LPS and bacterial DNA) and immune deficiency in people with HIV, including people with poor immune reconstitution on antiretroviral therapy (ART). However, these results have not ruled out the possibility that microbial translocation occurs as a result of HIV-induced immune deficiency, rather than playing a key role in causing it.
To try and gain a better understanding of the importance of microbial translocation in the pathogenesis of HIV infection, Daniel Douek has collaborated with researchers from the INSIGHT network to analyze samples from the Strategic Management of AntiRetroviral Therapy (SMART) trial. This trial randomized 5,472 people with HIV to either continuous or intermittent, CD4-guided ART, and the results showed that intermittent ART was associated with a doubling of the risk of illness and death compared to continuous treatment. During the study, 85 participants died, 142 developed major cardiovascular disease events, and 100 developed AIDS-defining events. Of these participants, 74, 120, and 81, respectively, had samples available.
Douek and colleagues used a case control study design to evaluate whether any of a suite of different markers of microbial translocation showed associations with these clinical outcomes. The same case control study design has previously been used to analyze the data from SMART, revealing a highly significant association between levels of inflammatory biomarkers and mortality. The biological markers assessed in the new study were: intestinal fatty acid binding protein (a marker of damage to cells of the gut wall called enterocytes), the bacterial product lipopolysaccharide (LPS), bacterial DNA (16S rDNA), anti-LPS antibodies (endotoxin core IgM antibody or EndoCAb) and soluble CD14 (sCD14). CD14 is a molecule expressed on monocytes that is known to be shed as a result of stimulation by LPS.
The only marker that showed a correlation with a clinical outcome was sCD14; higher levels were significantly associated with an increased risk of mortality. Levels of sCD14 also correlated with the inflammatory biomarkers that have previously been associated with mortality risk in SMART. The study authors offer a variety of possible reasons why other markers of microbial translocation were not associated with clinical outcomes, and argue strongly that elevated levels of sCD14 represent a consequence of microbial translocation (even though the other markers did not correlate with sCD14). There are, however, alternate possibilities that might explain elevated sCD14 levels that are not discussed in the paper. Specifically, alpha interferon has been reported to increase levels of sCD14, and levels of this cytokine are increased in HIV infection. Furthermore, a study published last year that looked for evidence of monocyte stimulation by LPS in HIV found that the gene expression patterns of these cells were more consistent with stimulation by alpha interferon, not LPS (abstract included below). So while Douek and colleagues write: “these observations are consistent with a model in which HIV infection causes ongoing damage to the gut mucosa, leading to increased microbial translocation, increased systemic inflammation, and increased mortality,” this interpretation of the data seems debatable, as the elevated levels of sCD14 may not necessarily be explained solely by microbial translocation.
Coincidentally, the current issue of the Journal of Infectious Diseases includes a letter from several researchers (Andrew Redd, Ronald Gray and Thomas Quinn) highlighting the uncertainty regarding whether microbial translocation is a cause or consequence of HIV pathogenesis. They argue that the current evidence favors the view that “increased microbial translocation and LPS levels are a consequence of advanced HIV-1 disease and AIDS.” The letter closes by stressing the need for additional longitudinal studies to fully resolve the issue.
J Infect Dis. 2011 Jan 20. [Epub ahead of print]
Sandler NG, Wand H, Roque A, Law M, Nason MC, Nixon DE, Pedersen C, Ruxrungtham K, Lewin SR, Emery S, Neaton JD, Brenchley JM, Deeks SG, Sereti I, Douek DC; for the INSIGHT SMART Study Group.
Background. Chronic human immunodeficiency virus (HIV) infection is associated with intestinal permeability and microbial translocation that contributes to systemic immune activation, which is an independent predictor of HIV disease progression. The association of microbial translocation with clinical outcome remains unknown. Methods. This nested case-control study included 74 subjects who died, 120 of whom developed cardiovascular disease and 81 of whom developed AIDS during the Strategies for Management of Anti-Retroviral Therapy (SMART) study with matched control subjects. Intestinal fatty acid binding protein (I-FABP), lipopolysaccharide (LPS), soluble CD14 (sCD14), endotoxin core antibody (EndoCAb), and 16S ribosomal DNA (rDNA) were measured in baseline plasma samples. Results. Subjects with the highest quartile of sCD14 levels had a 6-fold higher risk of death than did those in the lowest quartile (95% confidence interval, 2.2-16.1; P<.001), with minimal change after adjustment for inflammatory markers, CD4(+) T cell count, and HIV RNA level. No other marker was significantly associated with clinical outcomes. I-FABP, LPS, and sCD14 were increased and EndoCAb was decreased in study subjects, compared with healthy volunteers. sCD14 level correlated with levels of IL-6, C-reactive protein, serum amyloid A and D-dimer. Conclusions. sCD14, a marker of monocyte response to LPS, is an independent predictor of mortality in HIV infection. Therapeutic attenuation of innate immune activation may improve survival in patients with HIV infection.
J Infect Dis. (2011) 203 (5): 744-745.
Andrew D. Redd1, Ronald H. Gray2 and Thomas C. Quinn1
1 Laboratory of Immunoregulation, Division of Intramural Research National Institute of Allergies and Infectious Diseases, National Institue of Health, Baltimore, Maryland
2 Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland
AIDS. 2010 Jun 19;24(10):1415-23.
Rempel H, Sun B, Calosing C, Pillai SK, Pulliam L.
Department of Laboratory Medicine, San Francisco Veterans Affairs Medical Center, San Francisco, California, USA.
OBJECTIVES: HIV-1 infection dysregulates the innate immune system and alters leukocyte-gene expression. The objectives were two fold: to characterize the impact of HIV-1 infection on peripheral monocyte gene expression and to identify the predominant factor(s) responsible for altered gene expression.
DESIGN AND METHODS: In a cross-sectional study (n = 55), CD14 monocytes were isolated from 11 HIV-1 seronegative controls, 22 HIV-1 seropositive individuals with low-viral loads (LVL) and 22 HIV-1 seropositive individuals with high-viral loads (HVL). Monocyte gene expression data were collected for control, LVL and HVL individuals using high-density microarrays. We evaluated three HIV-1 disease-related peripheral factors, interferon (IFN)-alpha, IFN-gamma and lipopolysaccharide (LPS) as candidates causing monocyte dysregulation, by comparing gene expression profiles between study individuals and monocytes treated with these factors in vitro. Plasma from HIV-1 positive individuals was quantified for LPS and soluble CD14.
RESULTS: Monocytes from HIV-1-infected individuals with viral loads above 10,000 RNA copies/ml (HVL) displayed an activated phenotype. Characterization of gene expression revealed an ongoing immune response to viral infection including inflammation and chemotaxis. Gene expression analysis of in-vitro-treated HIV-1 seronegative monocytes with IFN-alpha, IFN-gamma or LPS demonstrated that IFN-alpha most accurately recapitulated the HIV-1 HVL profile. No LPS-induced gene expression signature was detected even in HIV-1 individuals with the highest LPS and sCD14 levels.
CONCLUSION: Monocyte gene expression in individuals with HIV-1 viremia is predominantly due to IFN-alpha, whereas individuals with LVL have a nonactivated phenotype. In monocytes, there was no discernible expression profile linked to LPS exposure.