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The synergistic action of the cytostatic drug cyclophosphamide (CP) and fragmented exogenous DNA causes illness and death in mice (Dolgova et al., 2011–2013). The observed “delayed death” effect was most clearly pronounced when the DNA preparation was administered 18 to 30 h after CP treatment. This time span was termed the “death window.” It was found that injections of exogenous DNA result in a sustained increase in bone-marrow cell (BMC) apoptosis, which occurs throughout the time of DNA administration (18–30 h). Exogenous DNA, both allogeneic and belonging to various taxa, induces BMC apoptosis. Plasmid DNA has the greatest effect on apoptosis induction. The analysis of reduction and restoration of the BMC subpopulations as the mice progressed to death revealed a virtually complete loss of the 12–20-μm fraction of the cell population (about 3–4% vs. 35–40% in the control), which corresponds to the maximum leukopenia on day 3 after CP treatment. However, the relative number of CD34+ hematopoietic stem cells (HSCs) from day 15 and until the end of the observation constituted 1.2–1.4%, which corresponds to the wild-type range. Comparison of BMC smears from the sternal bone marrow of the CP and CP + DNA groups of mice indicated that the BMC populations isolated from CP + DNA animals lack young committed lymphopoiesis progenitor cells. Moreover, the affected mice had immature blast cell types in their blood, which was never observed in healthy or CP-treated mice. Pathological and morphological analyses show that starting from posttreatment day 9, mice that received CP + DNA preparations displayed pronounced morphological changes in their lungs, liver, pancreas, central and peripheral immune system organs, and brain. Most of the pathological changes observed are consistent with a severe inflammatory response. This suggestion was proven by structural equivalents of functional involution of lymphoid organs, such as the thymus, spleen, and lymph nodes. We speculate that the death of treated animals resulted from multiple organ dysfunctions caused by accidental involution of lymphoid organs and the systemic inflammatory response syndrome, both associated with injections of fragmented exogenous DNA into experimental animals within the “death window,” which corresponds to the final step in the repair of the majority of CP-induced double-strand breaks.

Injections of exogenous DNA combined with a cytostatic agent cyclophosphamide (CP) cause illness and death in experimental mice. This phenomenon is referred to as delayed death. It has been found that fragments of exogenous DNA reach the bone marrow and enter the bone marrow cells (BMCs) 1–5 min after injections. Fragments of exogenous DNA are captured from culture medium by BMCs generated ex vivo. After joint incubation with BMCs of mice, the fragments of exogenous DNA are internalized into internal compartments in a nondegraded form. Up to 1800 kb of nucleic acid material can be present in each cell of BMCs generated ex vivo and treated with fragments of exogenous DNA. The fragments of exogenous DNA internalized in BMCs generated ex vivo of both intact and CP pretreated mice become circularized. In the case of intact mice, the fragments of exogenous DNA can form high-molecular weight structures in vivo. It is suggested that the exogenous fragments localized in BMC nuclei integrate into chromosome(s) of the recipient mouse genome when treated with CP and exogenous DNA.