Differential patterns of serum Fas ligand levels in blood donors

Fas ligand (FasL), a member of the tumor necrosis factor superfamily, induces apoptosis in cells expressing the receptor Fas/Apo-1. The apoptosis of T cell induced by Fas/FasL binding serves to downregulate the immune response. The serum FasL levels are reported to be increased in patients with various clinical conditions such as viral infections, malaria, tuberculosis, and autoimmune disorders. There are also reports of deranged immune activation including raised FasL levels in certain geographic areas in apparently healthy persons. During one study involving blood levels of various cytokines in neurosurgical patients, an elevated serum FasL level in some control samples (apparently healthy blood donors) was observed that was found out to be reactive for human immunodeficiency virus enzyme-linked immunosorbent assay (HIV-ELISA). This prompted us to further investigate the serum FasL levels in HIV-ELISA– reactive and nonreactive blood donors. We are here presenting our initial observation of differential patterns of serum FasL reactivity in blood donors with differing HIV antibody reactivity. This study was performed at a tertiary care referral center based blood bank over a 14-month duration (January 2006 to March 2007). Serum samples of all HIV-ELISA– (HIV-1 and -2 antibody, Biotest, Dreieich, Germany) reactive blood donors were stored frozen at -80°C for FasL determination. These samples were further confirmed by commercially available Western blot (WB) Assay (J Mitra, New Delhi, India) using World Health Organization criteria for interpretation of WB results. Serum FasL was measured by ELISA development kit (Duoset, R&D Systems, Minneapolis, MN; sensitivity, 31.25 pg/mL) as per the manufacturer’s instructions. Serum FasL levels from 50 ageand gender-matched blood donors (not reactive for HIV-ELISA) were taken as control. For comparison between-groups U test was performed using computer software (SPSS, Version 12.0, SPSS, Inc., Chicago, IL). A p value of less than 0.05 was considered significant. During the study period a total of 58 blood donors (from 20,169 whole-blood donations) reacted with HIVELISA, of which only 12 were positive by WB assay (2 were indeterminate on WB). The pattern of serum FasL levels in donors with clear-cut results on WB (WB-positive and WB-negative) is shown in Fig. 1. There was a significant increase (p < 0.001) in serum FasL levels of HIV-ELISA– reactive blood donors compared to normal controls (median FasL levels of 345 pg/mL [range, 170-620 pg/mL] in HIV-ELISA–reactive but WB-negative blood donors compared to median FasL levels of 61.2 pg/mL [range, nondetectable to 380 pg/mL] in normal controls). However, serum FasL levels (median, 107.5 pg/mL; range, 57-195 pg/mL) in confirmed HIV-positive blood donors (ELISA-reactive and WB-positive) were comparable to normal controls but significantly (p < 0.001) lower than HIV-ELISA–reactive but WB-negative blood donors. The FasL values of 2 indeterminate samples fell in confirmed HIV-positive group (data not shown). In our study, we observed no difference in serum FasL levels between normal controls and confirmed HIVpositive individuals unlike elevated serum FasL levels in HIV patients reported previously. There was a significant correlation between ELISA false-reactive samples and increased serum FasL levels indicating a common cause for the occurrence of both. We at this point are not able to assign any cause but it may be related to latent infections including tuberculosis, malaria, or subclinical autoimmune conditions, etc., which are known to increase FasL levels and cause false-reactive ELISA. In conclusion, our findings suggest that serum FasL may help in identifying false ELISA-reactive blood units. Further studies with larger sample size are warranted to evaluate this phenomenon in relation to HIV serostatus.