Increased platelet and microparticle activation in HIV infection: upregulation of P-selectin and tissue factor expression

Abstract

Objective: HIV-1-infected patients have an increased risk for atherothrombosis and cardiovascular disease, but the mechanism behind these risks is poorly understood. We have previously reported that expression of tissue factor (TF) on circulating monocytes is increased in persons with HIV infection and that TF expression is related to immune activation, to levels of HIV in plasma, and to indices of microbial translocation. In this study, we explore the activation state of platelets in HIV disease.

Methods: Here, using flow cytometry-based assays, we measured platelet and platelet microparticle (PMP) activation in samples from HIV-1-infected donors and controls.

Results: Platelets and PMPs from HIV-1-infected patients are activated (as reflected by expression of CD62 P-selectin) and also more frequently expressed the procoagulant TF than did platelets and PMPs obtained from controls. Expression of these proteins was directly related to expression of TF on monocytes, to markers of T-cell activation (CD38 and HLA-DR), and to plasma levels of soluble CD14, the coreceptor for bacterial lipopolysaccharride. Platelet and microparticle expression of TF was not related to plasma levels of HIV but expression of P-selectin was related to plasma levels of HIV; neither TF nor P-selectin expression was related to CD4 T-cell count.

Conclusions: Platelets and microparticles are activated in HIV infection, and this activated phenotype may contribute to the increased risk for cardiovascular and thrombotic events in this population although a role for other confounding cardiovascular risks cannot be completely excluded.

Figure 1. Platelets/microparticles from HIV infected patients have an activated phenotype

A) Platelets/microparticles are defined by their low complexity and small size in plasma and by expression of CD42b. Representative histograms and summary data among platelets/microparticles from healthy controls (N=18), patients with controlled viremia (HIV RNA<400 copies/mL, N=23), and uncontrolled viremia (HIV RNA>400 copies/mL, N=18) for surface expression of B) CD62P and C) tissue factor. The proportions of platelet/PMP that expressed CD62P in each of our patient groups were significantly different from the proportion in controls (p<0.001 for both). Likewise, the proportions of TF+ platelets/PMPs in each of our patient groups were significantly different from the proportion of TF+ platelets/PMPs in controls (p>0.001 for both) D) The proportions of TF+ platelets/microparticles and TF+ monocytes are directly related (r=0.4842 p=0.0078).

Figure 1. Platelets/microparticles from HIV infected patients have an activated phenotype

A) Platelets/microparticles are defined by their low complexity and small size in plasma and by expression of CD42b. Representative histograms and summary data among platelets/microparticles from healthy controls (N=18), patients with controlled viremia (HIV RNA<400 copies/mL, N=23), and uncontrolled viremia (HIV RNA>400 copies/mL, N=18) for surface expression of B) CD62P and C) tissue factor. The proportions of platelet/PMP that expressed CD62P in each of our patient groups were significantly different from the proportion in controls (p<0.001 for both). Likewise, the proportions of TF+ platelets/PMPs in each of our patient groups were significantly different from the proportion of TF+ platelets/PMPs in controls (p>0.001 for both) D) The proportions of TF+ platelets/microparticles and TF+ monocytes are directly related (r=0.4842 p=0.0078).

Figure 2. The proportions of platelets/microparticles expressing P-selectin or tissue factor are related to plasma levels of sCD14

Plasma samples were thawed in batch and levels of sCD14 were measured. In samples from all donors A) the proportion of P-selectin positive platelets/microparticles is directly related to plasma levels of sCD14, r=0.781 p<0.001; and B) the proportion of tissue factor positive platelets/microparticles is directly related to plasma levels of sCD14, r=0.3872 p=0.026.

Figure 3. The proportions of platelets/microparticles expressing P-selectin or tissue factor are related to markers of T cell activation

CD8+ T lymphocytes were identified by size, complexity, and by expression of CD3 and CD8, and markers of activation (CD38 and HLA-DR) were measured on phenotypically gated cells. In samples from all donors A) the proportion of P-selectin positive platelets/microparticles is directly related to CD8+ T cell activation, r=0.6308 p=0.0006; and B) the proportion of TF positive platelets/microparticles is directly related to CD8+ T cell activation, r=0.4454 p=0.0175.