Role of nitric oxide in the regulation of immune responses during rabies virus infection in mice

Rabies virus (RABV) stimulates nitric oxide (NO) production, which either triggers T cell differentiation or suppresses T cell function depending on its concentration. Herein, we assessed the potential role of NO in regulation of immune responses during RABV infection in mice model. The experimental animals were divided into four groups and 100LD50 of challenge virus standard (CVS) strain of RABV was inoculated intracerebrally on day 0 and subsequently aminoguanidine (AG; inducible nitric oxide synthase inhibitor) was injected intraperitoneally twice a day, up to 6 days. The samples were collected at 2, 4, 6, 8, 9, 10 and 12 days post infection (DPI). The immune cells including CD4+, CD8+ T lymphocytes and natural killer (NK) cells were estimated from peripheral blood mononuclear cells (PBMCs) and splenocytes. Serum total NO concentration, histopathology, immunohistochemistry, direct fluorescent antibody technique and TUNEL assay was performed. Infection with CVS resulted in significant early increase in CD4+, CD8+ and NK cells in blood and spleen until 2 DPI. From 4 DPI onwards significant reduction was noticed in these parameters which coincided with increased NO on 4 DPI, rising to maximum on 8 DPI, until their death on 10 DPI. Conversely, the CVS-AG treated group showed lower levels of NO and increased number of CD4+, CD8+ and NK cells. Increased number of cells in blood and spleen coincided with increased survival time, delayed development of clinical signs, reduced viral load and less apoptotic cells. NO played important role in regulation of immune responses during RABV infection. The findings of present study confirmed the role of NO and/or iNOS using iNOS inhibitor (aminoguanidine) in immune response during RABV infection, which would further help in understanding the virus immunopathogenesis with adoption of newer antiviral strategies to counter the progression of disease.