ERVK activity in CD8⁺ T cell immune cell compartment in patients with ALS

Endogenous retrovirus-K (ERVK) expression has been associated with Amyotrophic Lateral Sclerosis (ALS), and its viral proteins can be detected in affected brain and spinal cord tissues. Despite confirmation of ERVK load in the blood of patients with ALS, few studies have examined ERVK protein expression in immune cells. ERVK produces an enzyme called integrase (IN), which can cause DNA damage during the integration of viral DNA into the host genome. Given that genomic instability is a hallmark of ALS, we hypothesized that the ERVK IN enzyme may also be expressed in lymphoid and myeloid-derived immune cells of patients. Peripheral blood mononuclear cells (PBMC) were isolated from blood specimens using Ficoll isolation, and either flash-frozen for western blot analyses or affixed onto slides using the cytospin technique for subsequent confocal microscopy analysis. Image analysis of confocal micrographs revealed that ERVK IN expression was significantly elevated in CD3⁺CD8⁺ T cells from a subset of patients with ALS as compared to controls. CD3⁺CD8⁺ T cells in ALS exhibit enhanced number and size of ERVK IN puncta within the nucleus and at the plasma membrane. The DNA damage load, as measured by marker γH2AX, was strongly associated with ERVK IN levels in both controls and patients with ALS. Stratification of molecular data based on clinical parameters showed an association of elevated ERVK IN load in CD8⁺ T cells from patients with ALS with lower ALSFRS-R and higher King's scores, as well as a significant decline in lung function metrics. In bulk PBMC from patients with ALS, ERVK IN was associated with expression of immune checkpoint marker PD-1, but not T cell exhaustion marker TOX. ERVK IN in CD14⁺CD11b⁺ myeloid cells was also elevated, with ERVK⁺ cells exhibiting notable membrane ruffling typical of immune cell activation and increased expression of HLA-DR. However, ERVK IN expression in monocytes was not correlated with clinical metrics in patients with ALS. This work points to the use of ERVK IN in CD8⁺ cytotoxic T cells as a blood biomarker for ALS clinical trials, especially those focused on testing the efficacy of antivirals as a therapeutic strategy for ALS.