Preclinical Evaluation of Flt3 Ligand to Improve T Cell Adaptive Immune Response During Sepsis

Abstract

BackgroundSepsis is a leading cause of death in the United States. Severely burned patients are highly susceptible to opportunistic infections and sepsis, leading to multi‐organ failure and death. Studies by Hotchkiss and colleagues show that T cell dysfunction significantly contributes to impaired immune response during sepsis in non‐burn subjects. However, the changes in T cell adaptive immunity after burn wound sepsis have not been investigated in detail. We used a clinically relevant mouse model of burn wound infection to assess adaptive immune system function. Furthermore, we evaluated the cytokine, Fms‐like tyrosine kinase‐3 ligand (Flt3L), to improve T cell adaptive immunity post burn wound sepsis. Flt3L is known to stimulate the expansion of hemopoietic progenitors, particularly dendritic cells.Methods8–10 weeks old male BALB/c mice were subjected to 35% total body surface area full thickness burn and the wound was infected with Pseudomonas aeruginosa on day 4 post‐burn. Flow cytometry was used to characterize the cell counts and phenotype of CD4+ and CD8+ T and dendritic cells in spleen and wound draining lymph nodes. Flt3L (10 μg, i.p.) was administered daily for 4 days following burn injury.ResultsWound infection led to severe sepsis in the mice as evidenced by detection of bacteria in blood and lungs, and multi‐organ injury (increased markers of liver and kidney injury) on day 2 after infection and mortality rate of approximately 80% by seven days. We observed a significant decline (>80%) in blood absolute lymphocyte count and CD4+ and CD8+ T cell counts in the spleen and burn‐draining lymph nodes of wound infected mice as compared to non‐infected burn mice on day 2 post infection. Flt3L significantly attenuated the infection induced decline in CD4+ and CD8+ T cell counts in the spleen. Wound infection also caused a decline in the CD11c+ dendritic cell numbers in lymph nodes. Flt3L stimulated the expansion of both the splenic and lymph node dendritic cells and protected against the dendritic cell loss in the lymph nodes. Furthermore, wound infection caused an increase in the expression of Programmed Death‐Ligand 1 (PD‐L1) expression on dendritic cells, F4/80+ macrophages and Ly6C+ inflammatory monocytes in the spleen. PD‐L1 is known to interact with its binding partner PD‐1 on T cell surface leading to T cell inhibition. Flt3L attenuated the increase in PD‐L1 expression and thus indirectly improves T cell function.ConclusionBurn wound sepsis causes significant adaptive immune system dysfunction and Flt3L represents a novel therapeutic cytokine to protect T cell function during sepsis and merits further investigation.Support or Funding InformationSupported by NIH R01 GM66885