Differentiation of naïve CD4 T cells into T helper (Th) 2 cells requires signaling through the T cell receptor and an appropriate cytokine environment. IL-4 is critical for such Th2 differentiation. We show that IL-2 plays a central role in this process. The effect of IL-2 on Th2 generation does not depend on its cell growth or survival effects. Stat5a^-/-cells show diminished differentiation to IL-4 production, and forced expression of a constitutively active form of Stat5a replaces the need for IL-2.In vivoIL-2 neutralization inhibits IL-4 production in two models. Studies of restriction enzyme accessibility and binding of Stat5 to chromatin indicate that IL-2 mediates its effect by stabilizing the accessibility of theIl4gene. Thus, IL-2 plays a critical role in the polarization of naive CD4 T cells to the Th2 phenotype.

It is widely accepted that females have superior immune responses than males, but the ways by which sex hormones may enhance T cell responses are still poorly understood. In the present study,we analyzed the effect of estrogens on CD4 T cell activation and differentiation after immunization with exogenous antigens. We show that administration of low doses of 17ß‐estradiol (E2) to castrated female mice results in a striking increase of antigen‐specific CD4 T cell responses and in the selective development of IFN‐γ‐producing cells. Quantitative assessment of the frequency of T cells bearing a public TCR ß chain CDR3 motif demonstrated that the clonal size of primary antigen‐specific CD4 T cells was dramatically increased in immune lymph nodes from E2‐treated mice. By usingmice with disrupted estrogen receptor (ER) α or ß genes, we show that ERα, but not ERβ, was necessary for the enhanced E2‐driven Th1 cell responsiveness. Furthermore, ERα expressionin hematopoietic cells was essential, since E2 effects on Th1 responses were only observed in mice reconstituted with bone marrow cells from ERα^+/+, but not ERα‐deficient mice. These results demonstrate that estrogen administration promotes strong antigen‐specific Th1 cell responses in a mechanism that requires functional expression of ERα in hematopoietic cells.

Mastitis caused byEscherichia coliandStaphylococcus aureusis a major pathology of dairy cows. To better understand the differential response of the mammary gland to these two pathogens, we stimulated bovine mammary epithelial cells (bMEC) with eitherE. colicrude lipopolysaccharide (LPS) or withS. aureusculture supernatant (SaS) to compare the transcriptomic profiles of the initial bMEC response. By using HEK 293 reporter cells for pattern recognition receptors, the LPS preparation was found to stimulate TLR2 and TLR4 but not TLR5, Nod1 or Nod2, whereas SaS stimulated TLR2. Biochemical analysis revealed that lipoteichoic acid, protein A and α-hemolysin were all present in SaS, and bMEC were found to be responsive to each of these molecules. Transcriptome profiling revealed a core innate immune response partly shared by LPS and SaS. However, LPS induced expression of a significant higher number of genes and the fold changes were of greater magnitude than those induced by SaS. Microarray data analysis suggests that the activation pathways and the early chemokine and cytokine production preceded the defense and stress responses. A major differential response was the activation of the type I IFN pathway by LPS but not by SaS. The higher upregulation of chemokines (Cxcl10,Ccl2,Ccl5andCcl20) that target mononuclear leucocytes by LPS than by SaS is likely to be related to the differential activation of the type I IFN pathway, and could induce a different profile of the initial recruitment of leucocytes. The MEC responses to the two stimuli were different, as LPS was associated with NF-κB and Fas signaling pathways, whereas SaS was associated with AP-1 and IL-17A signaling pathways. It is noteworthy that at the protein level secretion of TNF-α and IL-1β was not induced by either stimulus. These results suggest that the response of MEC to diffusible stimuli fromE. coliandS. aureuscontributes to the onset of the response with differential leucocyte recruitment and distinct inflammatory and innate immune reactions of the mammary gland to infection.

T cell numbers are maintained within narrow ranges in vivo . Introduction of naïve cells into lymphopenic environments results in proliferation and differentiation driven by the recognition of peptide/MHC complexes and by cytokine signaling. This process, often described as homeostatic proliferation, is here referred to as spontaneous proliferation. We show that, although the presence of memory CD4 T cells of broad repertoire efficiently inhibits proliferation/differentiation of naïve CD4 T cells, a memory population of similar size comprised of cells with a repertoire of limited diversity fails to do so, implying that cells of a given specificity prevent responses of cells of the same or related specificity. This finding suggests that the immune system has evolved mechanisms to attain a memory cell repertoire of great diversity independently of foreign antigens.