Single-cell analysis of in vivo cytokine response networks activated by infection [electronic resource]
Keywords: Microbiology and Immunology.

Cytokines are essential in the orchestration of effective innate and adaptive immune responses. Immunologists have focused extensively on identifying cytokines, and the cells that produce cytokines, yet there is still little understanding of the complex dynamics that govern cytokine responses in vivo during infection. Chapter 1 provides a general introduction to the cytokine signaling induced by innate and adaptive immune responses. Emphasis is placed on the activation of the signal transducers and activator of transcription (STAT) family of transcription factors by numerous cytokines because monitoring of STAT activation is a sensitive biochemical marker of the cytokine response. Chapter 2 describes the in vivo innate inflammatory response to poxvirus infection. Chapters 3 deals with adaptive immunity and specifically the cytokine responses elicited in vivo by antigen specific T cell activation. Chapter 2 presents experiments demonstrating that early after in vivo exposure to various microbe-associated molecular patterns, STAT1 and STAT3 activation is potently induced in splenic dendritic cells and T cells. Mice with targeted gene deletions were used to identify toll-like receptors (TLRs), interferons, and interleukin-6 (IL-6) as necessary for early STAT activation. These experiments with reductionist microbial ligands demonstrated the feasibility of analyzing the cytokine response to infection with an intact pathogen in vivo. Chapter 2 goes on to describe the innate cytokine response to viral infection in experiments comparing the immune response to two closely related poxviruses: vaccinia and ectromelia (mousepox). Early and robust STAT1 and STAT3 activation was found to strongly correlate with survival of poxvirus infection. In addition, different modes of innate immune recognition distinguished vaccinia from ectromelia infection with toll-like receptor 2 (TLR2) activation being unique to immune recognition of vaccinia and not of ectromelia. These results suggest that different modes of early innate immune recognition could play an essential role in the containment of poxvirus infections and underlie the strict host species specificity of poxviruses. Chapter 3 deals with the paracrine cytokine responses elicited by antigen specific T cell activation in vivo. Using a T cell receptor (TCR) transgenic model we found profound and sustained activation of STAT5 in the Foxp3+ T regulatory cells (T regs) of mice immunized with a model peptide antigen after the adoptive transfer of antigen specific T cells. STAT5 activation in T regulatory cells was found to be interleukin-2 (IL-2) dependent and IL-2 signals elicited T regulatory cell proliferation and enhanced suppressive functions. Infection with vaccinia virus yielded a similar pattern of STAT5 activation in T regulatory cells, and STAT5 signaling was found to be even more potent under conditions of preexisting immune memory. These experiments revealed that antigen specific T-cell activation is intrinsically linked via paracrine IL-2 production to the activation of the polyclonal T reg repertoire, a response mechanism that could prevent autoimmunity. Chapter 3 also describes the regulation of autocrine IL-2 signaling during antigen specific T cell activation. Surprisingly, following TCR stimulation in vivo DO11 T cells did not display potent STAT5 activation even though these cells produced IL-2 and expressed the high affinity IL-2 coreceptor CD25. Only upon secondary TCR stimulation was robust autocrine IL-2 dependent STAT5 activation observed. Overall, these experiments demonstrate the nuances of IL-2/STAT5 signaling across numerous innate and adaptive cells populations revealing that in vivo T regulatory cells are extremely sensitive to antigen specific T cell activation. Targeting IL-2 signals specifically to antigen specific T cells while blocking IL-2 activation of T regs could enhance the efficacy of vaccination.