Progressive multifocal leukoencephalopathy (PML) is an often lethal JC virus (JCV)-mediated infection , arising in immune-compromised individuals such as HIV+ or leukemia patients [2,3]. PML remained rare after its first description in 1958 , but, in the 1980s, the spread of HIV and occurrence of AIDS lead to a wave of PML cases  and fueled the field of PML research until today . Drugs that have recently brought considerable attention to PML, are, for example, efalizumab (anti-CD11a)  and natalizumab . Until September 2013, 399 cases of natalizumab-associated PML have been reported since the introduction of the drug in 2006 . Our group has recently introduced L-selectin (CD62L) as a potential biomarker to assess individual risk to develop PML in multiple sclerosis patients under treatment with natalizumab: patients, whose CD4+ T cells expressed low amounts of CD62L after cryoconservation were proposed to be at high risk for developing PML . There are a number of links between HIV pathogenesis and CD62L, as many studies have shown that there is a strong correlation between shedding of CD62L from immune cells and HIV infection [10–13]. It seemed therefore logical to transfer our research on CD62L as a risk marker for natalizumab-associated PML to the field of HIV, because a biomarker for PML risk would be of great help in daily clinical practice, either for treatment decisions or for close clinical monitoring, as clinical parameters can up to now not predict the occurrence of PML during HIV infection [14,15].
Our study focused on the expression of CD62L on peripheral blood mononuclear cells of HIV+ patients and its correlation with clinical parameters. This surface marker was assessed on viable cells that were previously frozen and thawed immediately before analysis (flow cytometry gating in Supplementary Figure 1, https://links.lww.com/QAD/A477). Of note, CD62L expression did not correlate with the CDC stage of the individual patients, with the percentage of CD4+ T cells among CD3+ cells or CD4+ T-cell counts. Concerning virus counts in the blood or cerebrospinal fluid, there was also no correlation with CD62L (Supplementary Figure 2, https://links.lww.com/QAD/A477). As shown previously, the percentage of CD62L-expressing cells among CD8+ T cells was lower than among CD4+ T cells, which can be attributed to the expanded population of terminally differentiated (CD62L negative) effector cells in CD8+ T cells (Fig. 1a). There was a distinct group of 15 patients with levels lower than 25%, which included nine of the 10 patients who had developed PML in the past (Fig. 1a). The 10 PML patients in our study had, therefore, significantly lower CD62L values on CD4+ T cells than the patients who had not developed PML, even though some samples were taken years after PML diagnosis (Fig. 1b). CD8+ T cells’ CD62L values did not correlate with PML incidence (Fig. 1c). There was also no correlation between CD62L expression and years of infection, lymphocyte counts, detectable brain atrophy, T2-weighted MRI abnormality, treatment regimes, or presence of polyneuropathy and opportunistic infections (data not shown).
Our study suggests that lowered CD62L levels (<25%) are a potential biomarker for identifying patients at risk for HIV-associated PML. Our group has previously suggested CD62L as such a marker in the case of treatment with the monoclonal antibody natalizumab . There was one major difference in this cohort in comparison to the approach in multiple sclerosis patients: all the HIV patients had already developed PML before the blood samples were taken, which means that we cannot make any statements on the CD62L values prior to PML diagnosis or development, which would be needed to validate this as a predictive biomarker. However, the underlying causes or predisposition for HIV-associated PML were still present in our patients at the time of blood drawing, suggesting similar immune phenotypic patterns in these patients before and after onset of PML, such as expression levels of CD62L. There was one PML patient in our study (patient #3), whose CD62L expression was unusually high and the CD4+ cell counts were increasing after the study blood draw from initially 191 to 336/μl after 1 year. We assume that both effects were specific results of his treatment, as he was the only PML patient treated with retrovir after diagnosis of PML [he was not on highly active antiretroviral therapy (HAART) before diagnosis of PML] and retrovir has the highest cerebrospinal fluid penetration and central nervous system penetration-effectiveness rank . Of note, all patients in this study were treated with HAART.
The study results are consistent with our hypothesis that CD4+ T cells of some patients turn ‘irresponsive’ against JCV. These patients would be inclined to develop PML and we propose that selective CD62L loss after cellular stress (in this case freezing and thawing of the cells) is a potential biomarker for this immune condition. CD62L would be useful as a marker, because conventional markers of disease severity such as CD4+ cell count cannot predict PML incidence [15,17]. We cannot ultimately say whether the loss of CD62L in this context is functionally responsible for the development of PML or simply an indicative biomarker for this specific immune status. A prospective validation and study design would be needed for this. However, our data together with previously published studies in the field of HIV concerning loss of CD62L on immune cells [10–13] are in nice alignment with our hypothesis that the correlation between CD62L and PML risk is functional and not coincidental. Specificity of the CD62L test during HIV infection is currently at 82.4% (six false-positives in 34 patients) and the sensitivity at 90% (nine true-positives and one false-negative). Of note, the six false-positive patients could still develop PML in the future, but we conservatively put them as ‘false’-positive. As a possible clinical implication, HIV patients could be tested repetitively for their expression of CD62L to follow expression and dynamics. A low expression (in our hands <25%) would suggest either a change of treatment regimens with subsequent re-testing or close clinical monitoring for PML-specific changes via MRI and neurological examination.
Future studies, prospective as well as retrospective, are warranted to confirm and validate our findings in larger cohorts, and data sets including samples gathered before onset of PML would be of utmost interest.
The authors would like to thank Barbara Wrobel, Verena Schütte, and Schumina Säuberlich for excellent technical assistance and the blood donors for their participation in our study. Heinz Wiendl and Nicholas Schwab contributed equally to this study.
Conflicts of interest
Funding: This study was funded by the Deutsche Forschungsgesellschaft (DFG) Grant CRC128/ B1 to N.S. and H.W., and the PML consortium to N.S. and H.W.
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