A novel H1N1 influenza A virus emerged in April 2009, and rapidly reached pandemic proportions. We report a retrospective observational case study of pathologic findings in 8 patients with fatal novel H1N1 infection at the University of Michigan Health Systems (Ann Arbor) compared with 8 age-, sex-, body mass index-, and treatment-matched control subjects. Diffuse alveolar damage (DAD) in acute and organizing phases affected all patients with influenza and was accompanied by acute bronchopneumonia in 6 patients. Organizing DAD with established fibrosis was present in 1 patient with preexisting granulomatous lung disease. Only 50% of control subjects had DAD. Peripheral pulmonary vascular thrombosis occurred in 5 of 8 patients with influenza and 3 of 8 control subjects. Cytophagocytosis was seen in all influenza-related cases. The autopsy findings in our patients with novel H1N1 influenza resemble other influenza virus infections with the exception of prominent thrombosis and hemophagocytosis. The possibility of hemophagocytic syndrome should be investigated in severely ill patients with H1N1 infection.

Using a model of established malignancy, we found that cyclophosphamide (Cy), administered at a dose not requiring hematopoietic stem cell support, is superior to low-dose total body irradiation in augmenting antitumor immunity. We observed that Cy administration resulted in expansion of tumor antigen-specific T cells and transient depletion of CD4+Foxp3+ regulatory T cells (Tregs). The antitumor efficacy of Cy was not improved by administration of anti-CD25 monoclonal antibody given to induce more profound Treg depletion. We found that Cy, through its myelosuppressive action, induced rebound myelopoiesis and perturbed dendritic cell (DC) homeostasis. The resulting DC turnover led to the emergence of tumor-infiltrating DCs that secreted more IL-12 and less IL-10 compared to those from untreated tumor-bearing animals. These newly recruited DCs, originating from proliferating early DC progenitors, were fully capable of priming T cell responses and ineffective in inducing expansion of Tregs. Together, our results show that Cy-mediated antitumor effects extend beyond the well-documented cytotoxicity and lymphodepletion and include resetting the DC homeostasis, thus providing an excellent platform for integration with other immunotherapeutic strategies.

Donor CD4+ T cells are thought to be essential for inducing delayed host tissue injury in chronic graft-versus-host disease (GVHD). However, the relative contributions of distinct effector CD4+ T cell subpopulations and the molecular pathways influencing their generation are not known. We investigated the role of the STAT3 pathway in a murine model of chronic sclerodermatous GVHD. This pathway integrates multiple signaling events during the differentiation of naive CD4+ T cells and impacts their homeostasis. We report that chimeras receiving an allograft containing STAT3-ablated donor CD4+ T cells do not develop classic clinical and pathological manifestations of alloimmune tissue injury. Analysis of chimeras showed that abrogation of STAT3 signaling reduced the in vivo expansion of donor-derived CD4+ T cells and their accumulation in GVHD target tissues without abolishing antihost alloreactivity. STAT3 ablation did not significantly affect Th1 differentiation while enhancing CD4+CD25+Foxp3+ T cell reconstitution through thymus-dependent and -independent pathways. Transient depletion of CD25+ T cells in chimeras receiving STAT3-deficient T cells resulted in delayed development of alloimmune gut and liver injury. This delayed de novo GVHD was associated with the emergence of donor hematopoietic stem cell-derived Th1 and Th17 cells. These results suggest that STAT3 signaling in graft CD4+ T cells links the alloimmune tissue injury of donor graft T cells and the emergence of donor hematopoietic stem cell-derived pathogenic effector cells and that both populations contribute, albeit in different ways, to the genesis of chronic GVHD after allogenic bone marrow transplantation in a murine model.