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Background: The use of inbred mice housed under standardized environmental conditions has been critical in identifying immuno-pathological mechanisms in different infectious and inflammatory diseases as well as revealing new therapeutic targets for clinical trials. Unfortunately, only a small percentage of preclinical intervention studies using well-defined mouse models of disease have progressed to clinically-effective treatments in patients. The reasons for this lack of bench-to-bedside transition are not completely understood; however, emerging data suggest that genetic diversity and housing environment may greatly influence muring immunity and inflammation. Results: Accumulating evidence suggests that certain immune responses and/or disease phenotypes observed in inbred mice may be quite different than those observed in their outbred counterparts. These differences have been thought to contribute to differing immune responses to foreign and/or auto-antigens in mice vs. humans. There is also a growing literature demonstrating that mice housed under specific pathogen free conditions possess an immature immune system that remarkably affects their ability to respond to pathogens and/or inflammation when compared with mice exposed to a more diverse spectrum of microorganisms. Furthermore, recent studies demonstrate that mice develop chronic cold stress when housed at standard animal care facility temperatures (i.e. 22-24°C). These temperatures have been shown alter immune responses to foreign and auto-antigens when compared with mice housed at their thermo-neutral body temperature of 30-32°C. Conclusions: Exposure of genetically diverse mice to a spectrum of environmentally-relevant microorganisms at housing temperatures that approximate their thermo-neutral zone may improve the chances of identifying new and more potent therapeutics to treat infectious and inflammatory diseases.

Background Hematopoietic stem cell transplantation is a potential cure for certain life-threatening malignant and nonmalignant diseases. However, experimental and clinical studies have demonstrated that pre-transplant myeloablative conditioning damages the gut leading to translocation of intestinal bacteria and the development of acute graft vs. host disease (aGVHD). The overall objective of this study was to determine whether administration of broad spectrum antibiotics (Abx) affects the onset and/or severity of aGVHD in lymphopenic mice that were not subjected to toxic, pre-transplant conditioning. Results We found that treatment of NK cell-depleted recombination activating gene-1-deficient (-NK/RAG) recipients with an Abx cocktail containing vancomycin and neomycin for 7 days prior to and 4 weeks following adoptive transfer of allogeneic CD4+ T cells, exacerbated the development of aGVHD-induced BM failure and spleen damage when compared to untreated–NK/RAG recipients engrafted with syngeneic or allogeneic T cells. Abx-treated mice exhibited severe anemia and monocytopenia as well as marked reductions in BM- and spleen-residing immune cells. Blinded histopathological analysis confirmed that Abx-treated mice engrafted with allogeneic T cells suffered significantly more damage to the BM and spleen than did untreated mice engrafted with allogeneic T cells. Abx-induced exacerbation of BM and spleen damage correlated with a dramatic reduction in fecal bacterial diversity, marked loss of anaerobic bacteria and remarkable expansion of potentially pathogenic bacteria. Conclusions We conclude that continuous Abx treatment may aggravate aGVHD-induced tissue damage by reducing short chain fatty acid-producing anaerobes (e.g. Clostridium, Blautia) and/or by promoting the expansion of pathobionts (e.g. Akkermansia) and opportunistic pathogens (Cronobacter).

Acute graft vs. host disease (aGVHD) is a major complication following hematopoietic stem cell transplantation. Preclinical and clinical studies suggest aGVHD may be potentiated by gut damage and translocation of intestinal microbiota following toxic, pre-transplant conditioning protocols. We recently reported that aGVHD-induced bone marrow (BM) failure and splenic hypoplasia develops in the absence of gut damage suggesting that intestinal bacteria may not be required for disease pathogenesis. Objective Determine whether prophylactic gut decontamination with broad spectrum antibiotics (Abx) affects the onset and/or severity of aGVHD-induced BM and spleen damage. Methods Syngeneic (Bl6) or allogeneic (Balb/c) CD4+CD25−T cells (5×106 cells) were injected into NK cell-depleted Bl6 RAG1−/− recipients. Prior to T cell transfer, RAG1−/− mice received water (ab libitum) containing aspartame (Asp) or an Abx cocktail containing Asp, neomycin and vancomycin for 7 days prior to and following T-cell transfer. Results Treatment of allogeneic mice with Abx reduced colonic bacterial load by more than 20-fold when compared to their Asp-treated counterparts. Abx treatment also resulted in large and significant reductions in BM- and spleen-residing T cells and myeloid cells as well as circulating erythrocytes, platelets and hematocrit when compared to Asp treated mice. These Abx-induced alterations were associated with significant increases (~4-fold) in serum IL-6 levels compared to Asp-treated mice. Conclusions Prophylactic Abx treatment exacerbates aGVHD-induced BM failure and spleen hypoplasia.

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