To the Editors:
Thrombocytopenia and increased levels of platelet activation markers are common in untreated HIV-infected individuals.1,2 Although low platelet counts usually correct after initiation of highly active antiretroviral therapy (HAART),1 little is known about the functional state of platelets in this scenario. Cellular microparticles are small (<1.5 μm), cell membrane-derived vesicular fragments released upon cellular activation or apoptosis of parental cells.3 Abnormal levels of platelet microparticles (PMP) are associated with several prothrombotic conditions.4-6 Expression of P-selectin (CD62P) on platelets is a well-recognized indicator of platelet activation.7 We sought to evaluate these markers in HIV-infected patients with optimal response to stable HAART regimens.
After proper approval and written informed consent, we enrolled adult (age ≥ 18 years), otherwise healthy HIV-infected patients receiving the same HAART for at least 6 months and who had HIV viral loads <50 copies/mL and CD4+ T-cell counts >200/mm3 in at least 2 separate measurements over that period of time. Patients were excluded if their platelet counts were <150,000 or >450,000/μL. A control group of healthy adults matched for age and sex was used for comparisons. The protocol for obtaining, processing, and measuring PMP has been described elsewhere.5 In brief, PMP were measured by flow cytometry using phycoerythrin-labeled anti-CD31 and fluorescein-isothiocyanate-labeled anti-CD42b (Pharmingen, San Diego, CA). Because CD31 occurs on platelets and endothelial cells but CD42b occurs only on platelets, PMP were defined as events CD31+/CD42b+. Values are reported as counts per microliter. CD31 also occurs on some leukocyte subsets, but they account for a negligible fraction of microparticles.5 We measured P-selectin expression as described previously.8 We used Mann-Whitney U and t test for comparisons of continuous variables, Fisher Exact test for comparisons of categorical variables, and Spearman rank correlation coefficients for assessment of correlations. All calculations were performed with Intercooled STATA 9.2 (College Station, TX).
Twenty-eight HIV-infected patients and 27 healthy controls were enrolled in the study. Both groups had comparable ages (42 ± 6.7 and 41 ± 6.9, respectively; P = 0.59). The groups also demonstrated similar sex and race distributions (P = 0.72 and 0.41, respectively). The mean CD4 count in the HIV-infected group was 550 (±195). Fifteen (57%) of these patients had history of at least one other previous antiretroviral regimen. The mean time receiving the current HAART was 21 (±10.5) months; and the mean time with undetectable HIV viral load was 13 (±9) months. All patients were receiving nucleoside reverse transcriptase inhibitors (NRTIs) as the backbone of their HAART regimens [lamivudine: 19, abacavir (ABC): 13, TDF: 12, FTC: 4, zidovudine (AZT): 11, stavudine: 4, didanosine: 1]. Protease inhibitors (PIs) complemented the therapy in 13 (46%) patients (lopinavir (LPV): 5, nelfinavir (NFV): 2, indinavir (IDV): 2, atazanavir (ATV): 2, fosamprenavir (FPV): 2; all boosted with RTV except for NFV- and one FPV-containing regimens), and nonnucleoside reverse transcriptase inhibitors did so in 15 (54%) of them (efavirenz: 10, nevirapine: 5).
PMP levels were significantly higher among HIV-infected subjects than controls [25.28 × 103 counts/μL, interquartile range (IQR): 17.89 × 103-36.18 × 103 versus 13.86 × 103 counts/μL, IQR: 99.84 × 103-20.74 × 103; P = 0.002; Fig. 1]. P-selectin (CD62P) levels, on the other hand, were similar among these groups (7.95 fluorescence intensity units (FIU), IQR: 4.05-15.92 versus 13.32 FIU, IQR: 4.91-23.02; P = 0.17; Fig. 1). We found no differences in the levels of PMP or CD62P when they were compared, within the HIV-infected group, between those receiving PI-based therapy or not; or those receiving ABC or not. Likewise, we found no relation between PMP levels and the duration of exposure to HAART or length of time with undetectable HIV viral load. Higher CD4+ T-cell counts, however, had a trend toward a negative correlation with PMP levels (rho = −0.32, P = 0.1).
Earlier studies documented heightened levels of PMP and platelet activation in untreated HIV-infected patients.2,9 Platelet activity, but not PMP levels, could be reduced after a short period (3 months) of antiretroviral therapy.2 Our results now show that when compared with healthy controls, HIV-infected patients with good response to longer and stable courses of HAART continue to express higher levels of circulating PMP despite comparable levels of CD62P. In this setting, PMP could be more sensitive markers-than CD62P-of residual increased platelet activity due to unrelenting effects of the infection (eg, low-level viral replication and chronic immune stimulation). The trend toward a negative correlation between PMP levels and CD4+ T-cell counts favors this option. Alternatively, increased levels of PMP may persist from processes independent of platelet activation, such as, for instance, apoptosis. HIV virus infects megakaryocytes10 and cells of the monocyte/macrophage lineage11; with the latter being the main scavengers of apoptotic microvesicles.12 Persistent subclinical HIV viral activity (not detected with the currently available methods) may increase the levels of PMP by promoting apoptosis of thrombocytes or impairing the clearance of their apoptotic products. Another possibility is a direct toxic effect of HAART on platelets. Nucleoside reverse trancriptase inhibitors are known to cause mitochondrial toxicity13 and the potential for this effect to trigger apoptosis has been recognized.14 Under this scenario, a positive link between levels of PMP and longer periods of HAART exposure would be expected but this was not apparent in our analysis.
PMP exert important procoagulant functions15 and their higher levels have been linked to the development of acute coronary syndromes (ACS).4,6,16 Hence, regardless of the mechanism involved in their release, PMP could contribute to the increased risk of ACS seen in the HIV-infected patients receiving HAART. PIs and ABC are the drugs more consistently associated with the occurrence of ACS in this population17 but we could not demonstrate any specific link between them and PMP levels.
Although small, our groups were homogenous in terms of preexisting clinical characteristics and response to HAART. Our cross-sectional measurements, however, prevents us from drawing finer conclusions on the temporal effect of HAART on the measured variables.
This study substantiates the concept of persistent platelet abnormalities in HIV-infected patients with good response to HAART. The causes of this phenomenon and its implications in the cardiovascular pathology of this population will require further investigations.
Supported by the Wallace H. Coulter Foundation.
Vicente F. Corrales-Medina, MD*
Jacques Simkins, MD*
Julio A. Chirinos, MD*
Jose A. Serpa, MD†
Lawrence L. Horstman, BSc‡
Wenche Jy, PhD‡
Yeon-Soong Ahn, MD‡
*Department of Medicine, Miller School of Medicine, University of Miami, Miami, FL
†Department of Medicine, Baylor College of Medicine, Houston, TX
‡Wallace Coulter Platelet Laboratory, Miller School of Medicine, University of Miami, Miami, FL
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