Increased tissue factor expression on circulating monocytes in chronic HIV infection: relationship to in vivo coagulation and immune activation

Abstract

HIV infection is associated with an increased risk of thrombosis; and as antiretroviral therapy has increased the lifespan of HIV-infected patients, their risk for cardiovascular events is expected to increase. A large clinical study found recently that all-cause mortality for HIV(+) patients was related to plasma levels of interleukin-6 and to D-dimer products of fibrinolysis. We provide evidence that this elevated risk for coagulation may be related to increased proportions of monocytes expressing cell surface tissue factor (TF, thromboplastin) in persons with HIV infection. Monocyte TF expression could be induced in vitro by lipopolysaccharide and flagellin, but not by interleukin-6. Monocyte expression of TF was correlated with HIV levels in plasma, with indices of immune activation, and with plasma levels of soluble CD14, a marker of in vivo lipopolysaccharide exposure. TF levels also correlated with plasma levels of D-dimers, reflective of in vivo clot formation and fibrinolysis. Thus, drivers of immune activation in HIV disease, such as HIV replication, and potentially, microbial translocation, may activate clotting cascades and contribute to thrombus formation and cardiovascular morbidities in HIV infection.

Figure 1

Exposure of human monocytes to TLR ligands, but not to IL-6, increases surface expression of the procoagulant TF. Whole blood was obtained from HIV-uninfected subjects and was exposed to LPS (50 ng/mL), flagellin (10 μg/mL), or IL-6 (30 μg/mL) for 3 hours. Surface expression of TF was measured on CD14⁺ monocytes by flow cytometry. (A) Gating strategy. (B) Representative histograms. (C) Summary data.

Figure 2

The proportion of monocytes expressing TF and plasma levels of bioactive TF are increased in HIV infection, and monocyte TF correlates with markers of immune activation. Whole-blood samples obtained from 60 HIV-infected donors and 19 healthy controls were examined for expression of TF on gated CD14⁺ monocytes and for markers of immune activation (HLA-DR and CD38) on gated CD8⁺ T cells by flow cytometry. (A) Representative histograms and dot plots. (B) Summary data among healthy controls, patients with controlled (HIV RNA < 400 copies/mL), and uncontrolled (HIV RNA > 400 copies/mL) viremia. (C) Bioactive TF was significantly higher in plasma samples from HIV-infected patients (n = 28) than in samples from uninfected controls (n = 8, P < .001). (D) Correlation between the proportion of TF⁺ monocytes and the proportion of CD8⁺ T cells that express activation markers (r = 0.61, P < .001).

Figure 3

TF expression is correlated with the magnitude of viremia and is weakly correlated with CD4⁺ T-cell counts. (A) Correlation between the proportion of TF⁺ monocytes and plasma levels of HIV RNA in patients (r = 0.57, P < .001). (B) Correlation between the proportion of TF⁺ monocytes and circulating CD4⁺ T-cell numbers in patients (r = −0.32, P = .014).

Figure 4

Plasma levels of the LPS coreceptor, CD14, are increased in HIV infection and correlate with the proportion of TF⁺ monocytes in patients with detectable viremia. Plasma was collected from whole-blood samples provided by HIV-infected (N = 46) and -uninfected donors (N = 18), and levels of sCD14 were measured. (A) sCD14 levels are higher in plasmas of HIV⁺ patients than in plasmas of healthy controls (P < .005). (B) Plasma levels of sCD14 correlate with the proportion of TF⁺ monocytes among all subjects (r = 0.51, P < .001). (C) Plasma levels of sCD14 correlate with the proportion of TF⁺ monocytes among patients with detectable viremia (r = 0.44, P = .042, n = 22).

Figure 5

Plasma D-dimer levels are increased in HIV infection and correlate with the proportion of monocytes expressing TF. Plasma was collected from whole-blood samples provided by HIV-infected (n = 38) and -uninfected donors (n = 18), and levels of D-dimers were measured. (A) Levels of D-dimers are higher in plasma samples from HIV-infected patients than in samples from HIV-uninfected controls (P = .032). (B) Plasma levels of D-dimers correlate with the proportion of TF⁺ monocytes in all samples tested (r = 0.37, P = .007).