Dynamics of cell-free DNA levels in the in vivo LPS-induced inflammation model

An increased concentration of extracellular cell free DNA (cfDNA) is a distinctive characteristic of pathologies that mainly occur in acute inflammation (myocardial infarction, sepsis, stroke, trauma). The increase of cfDNA in chronic inflammatory processes, oncological, autoimmune diseases is less significant and is mainly due to aberrant cell death processes. One of such diseases is systemic lupus erythematosus (SLE). It has recently been shown that, in addition to increased cfDNA concentration, the degree of inflammation can reflect the N/L index (neutrophil to lymphocyte ratio), being a simple and informative marker of disease activity in patients with SLE. The aim of the study was to study the dynamics of the level of cfDNA and the N/L index in the model of LPS-induced inflammatory response as observed in intact mice, and their relation to the phenotypic heterogeneity of model SLE. We used female hybrid mice (C57Bl/6xDBA/2) F1 and female DBA/2 mice at the age of 6-8 weeks. LPS of E. coli strain 111: B4 (Sigma) was injected intraperitoneally once at doses of 10 ng, 1 µg and 100 µg per mouse in PBS. The control group was injected with the appropriate volume of buffer. The TNFα-binding domain of the variola virus CRMB protein was used as an inhibitor of TNFα, which was administered 30 min before the introduction of LPS. The dynamics of the response to LPS was assessed after 4, 8, 11, 24 hours by the N/L index and the level of cfDNA; at the zero point, the parameters were determined before the introduction of LPS. A day after a single injection of LPS at a dose of 1 µg/mouse, a SLE model was induced on the same hybrid mice (double intravenous administration with an interval of 6 days of spleen cells of the DBA/2 line, 60-70 × 106 cells each). Three months later, with proteinuria of 3 mg/ mL or more, mice were assigned to the SLE^nephritis+ group, with a protein of less than 3 mg/mL, to the SLE^nephritis- group. Statistical processing of the results was carried out by nonparametric statistics using the Mann–Whitney test. Differences were considered statistically significant at p < 0.05. It was found that the change in the N/L index, as well as the change in the level of cfDNA, depends on the dose of LPS administered. It was shown that the level of cfDNA reaches its maximum after 8 and 11 hours after the introduction of LPS is reliably reduced when using the inhibitor TNFα. A retrospective analysis indicates that there is a definite relationship between the response of intact mice to LPS before induction of cGVHD, and their subsequent division into variants of SLE^{nephritis +} and SLE^{nephritis -} in the course of disease development.