In five patients from whom sera prior to PML diagnosis were available, antibody titres increased 5–10 months before PML diagnosis [61]. Methodological issues such as fluctuating serostatus around assay cut-points [52, 61] and false negative rates [60] argue for a refinement of assay procedures with better reproducibility in low-antibody reactivity ranges. Thus, a second-generation enzyme-linked immunosorbent assay (ELISA) with a reported sensitivity of 98% [62] was introduced; however, so far an independent validation is lacking. Using this refined assay, the possible value

of antibody reactivity for PML risk stratification was reported recently SB203580 in vivo as abstract. buy Torin 1 Whereas increased immunoreactivity to JCV prior to PML would be biologically plausible, more data are needed to corroborate these initial findings. Higher NAT plasma levels have been associated with lower body mass index and a supposedly higher risk for the development of PML, which needs to be further confirmed as a possible biomarker feasible for clinical routine [44]. Host factors promoting PML development include the determination of immunocompetence. It has been shown conclusively that both CD4+ and CD8+

T cells are important in the immune response to JCV and containment of PML [48, 63]. Investigation of the role of CD4+ T cells has demonstrated a lacking or even anti-inflammatory interleukin (IL)-10 response to JCV in a small number of PML patients [64]. Intracellular adenosine triphosphate

(ATP) levels as a functional parameter of T cell function were decreased Mannose-binding protein-associated serine protease in CD4+ T cells both after long-term NAT treatment and PML of different aetiology [65]. However, this assay was confronted with pre-analytical difficulties, so far impeding application in larger validating studies or clinical routine, as shown by analysis of STRATA samples (Natalizumab Re-Initiation of Dosing; ClinicalTrials.gov NCT00297232) that could not confirm ATP decrease in five pre-PML samples [66]. However, heterogeneous intervals of testing before PML onset may have influenced these results. It may be hypothesized that individual courses of ATP levels are more critical than absolute ATP level, and that a critical time-point of ATP decrease before PML onset has to be determined. Recently, a lower proportion of L-selectin-expressing CD4+ T cells was associated with higher PML risk in NAT-treated MS patients (n = 8). Further validation as a potential biomarker for PML risk stratification is warranted [67]. The determination of its biological plausibility remains unclear thus far, as it might express the general activation status of the peripheral immune system or a defective T cell response to JCV infection on different levels [67].