Although interferon γ (IFN-γ) secretion is essential for control of most intracellular pathogens, host survival often also depends on the expression of interleukin 10 (IL-10), a cytokine known to counteract IFN-γ effector functions. We analyzed the source of regulatory IL-10 in mice infected with the protozoan parasite Toxoplasma gondii. Unexpectedly, IFN-γ–secreting T-bet+Foxp3− T helper type 1 (Th1) cells were found to be the major producers of IL-10 in these animals. Further analysis revealed that the same IL-10+IFN-γγ population displayed potent effector function against the parasite while, paradoxically, also inducing profound suppression of IL-12 production by antigen-presenting cells. Although at any given time point only a fraction of the cells appeared to simultaneously produce IL-10 and IFN-γ, IL-10 production could be stimulated in IL-10−IFN-γ+ cells by further activation in vitro. In addition, experiments with T. gondii–specific IL-10+IFN-γ+ CD4 clones revealed that although IFN-γ expression is imprinted and triggered with similar kinetics regardless of the state of Th1 cell activation, IL-10 secretion is induced more rapidly from recently activated than from resting cells. These findings indicate that IL-10 production by CD4+ T lymphocytes need not involve a distinct regulatory Th cell subset but can be generated in Th1 cells as part of the effector response to intracellular pathogens.

Schistosoma mansoni eggs contain factors that trigger potent Th2 responses in vivo and condition mouse dendritic cells (DCs) to promote Th2 lymphocyte differentiation. Using an in vitro bystander polarization assay as the readout, we purified and identified the major Th2-inducing component from soluble egg extract (SEA) as the secreted T2 ribonuclease, omega-1. The Th2-promoting activity of omega-1 was found to be sensitive to ribonuclease inhibition and did not require MyD88/TRIF signaling in DCs. In common with unfractioned SEA, the purified native protein suppresses lipopolysaccharide-induced DC activation, but unlike SEA, it fails to trigger interleukin 4 production from basophils. Importantly, omega-1–exposed DCs displayed pronounced cytoskeletal changes and exhibited decreased antigen-dependent conjugate formation with CD4⁺ T cells. Based on this evidence, we hypothesize that S. mansoni omega-1 acts by limiting the interaction of DCs with CD4⁺ T lymphocytes, thereby lowering the strength of the activation signal delivered.

The concept that IL-4 is the primary signal for Th2 lymphocyte differentiation has recently been put in doubt by studies in which the production of Th2-associated cytokines was detected in mice deficient in IL-4 synthesis or IL-4R triggering. In this study, we formally demonstrate by single cell analysis that CD4⁺ lymphocytes with a classical Th2 phenotype (IL-4⁺, IL-5⁺, IFN-γ⁻, IL-2⁻) develop in significant numbers in helminth-infected mice deficient in either IL-4R α-chain or Stat6. While an expanded population of Th1 (IL-4⁻, IL-5⁻, IFN-γ⁺, IL-2⁺) lymphocytes was observed in the same animals, surprisingly, cells with a mixed Th0 cytokine pattern were rare. The cytokine production phenotypes of the Th1 and Th2 subpopulations generated in infected Stat6-deficient mice were unaffected by in vitro neutralization of endogenous IL-4 or IFN-γ. Nevertheless, while addition of exogenous rIL-12 resulted in transitory IFN-γ production by Th2 lymphocytes from both wild-type and Stat6-deficient mice, IL-4 synthesis was preserved in the former, but temporarily ablated in the latter cells. Importantly, IL-4⁺ IFN-γ⁻ and IL-4⁻ IFN-γ⁺ populations similar to those arising in helminth-infected Stat6-deficient mice could also be generated in vitro by repetitive polyclonal stimulation of CD4⁺CD62L^high lymphocytes from uninfected mice of the same strain. Together, the results of these single cell analysis experiments demonstrate that IL-4R/Stat6 signaling, while influencing the final frequency of Th2 lymphocytes, is not essential for Th2 cell development, and suggest that this pathway has a previously unrecognized function in stabilizing Th2 populations once they have emerged.

Type 1 T helper (Th1) cells play a critical role in host defense against intracellular pathogens and in autoimmune diseases by producing a key inflammatory cytokine interferon (IFN)–γ; some Th1 cells can also be antiinflammatory through producing IL-10. However, the molecular switch for regulating the differentiation of inflammatory and antiinflammatory Th1 cells is still elusive. Here, we show that Bhlhe40-deficient CD4 Th1 cells produced less IFN-γ but substantially more IL-10 than wild-type Th1 cells both in vitro and in vivo. Bhlhe40-mediated IFN-γ production was independent of transcription factor T-bet regulation. Mice with conditional deletion of Bhlhe40 in T cells succumbed to Toxoplasma gondii infection, and blockade of IL-10 signaling during infection rescued these mice from death. Thus, our results demonstrate that transcription factor Bhlhe40 is a molecular switch for determining the fate of inflammatory and antiinflammatory Th1 cells.