These signals are mainly provided by members of the B7-family including CD80 and CD86. However, macrophages

can also inhibit T-cell activation by release of inhibitory cytokines such as IL-10 and TGF-β or metabolic starvation due to depletion of tryptophan by indoleamine-2,4-dioxygenase 19 and depletion of arginin by nitric oxide synthase (iNOS) or Arg1 Sunitinib 20. In addition, macrophages can suppress T cells by direct cell–cell contact via expression of ligands for inhibitory receptors. B7-H1 (PD-L1) and B7-DC (PD-L2) are two members of the B7-family, which bind to programmed death 1 (PD-1), an inhibitory receptor on T cells. Similar to its effects on cytokine production, chitin may modulate expression Sorafenib of costimulatory ligands on macrophages and thereby regulate the efficiency of T-cell activation, differentiation and proliferation. However, this possibility has not been examined experimentally. To address this point directly, we determined

whether chitin modulates Th2 polarization and T-cell proliferation using adoptive transfers and coculture systems. We observed that chitin reduced the expansion of antigen-specific CD4+ T cells in vivo. Chitin-exposed macrophages upregulated B7-H1 independently of signaling via TLR or Stat6 and blocked T-cell proliferation in a cell–cell contact-dependent manner. Inhibition of T-cell proliferation was not observed with cells from B7-H1-deficient mice which indicates that chitin inhibits T-cell proliferation indirectly by inducing expression of B7-H1 on macrophages. Intranasal administration of chitin particles induces early recruitment of macrophages and neutrophils followed later by basophils and eosinophils 9, 18. As basophils express large amounts of IL-4 and have recently been shown to initiate Th2 differentiation in response to the pro-allergic protease papain, Interleukin-3 receptor we sought that chitin-induced basophil recruitment might result in priming and expansion of Th2 cells in the lung 21, 22. Therefore, we determined whether intranasal chitin administration leads to enhanced Th2-cell differentiation

in the lung and draining LN. To visualize Th2-cell differentiation, we used IL-4 reporter mice (4get mice), which were crossed to DO11.10 TCR-tg mice so that the OVA-specific T-cell responses could be analyzed. BALB/c mice were reconstituted with 106 TCR-tg cells from DO11.10/4get mice followed by intranasal administration of OVA protein in the presence or absence of small (20–50 μm) chitin particles. Administration of OVA induced expansion of TCR-tg cells (KJ1-26+ cells) in lung and LN, whereas T-cell expansion was five-fold reduced in mice which received OVA plus chitin (Fig. 1A and B). In addition, Th2-cell differentiation was induced only in OVA but not in OVA/chitin-treated mice (KJ1-26+IL-4/eGFP+ cells in Fig. 1A). Therefore, chitin did not enhance but rather inhibited the Th2 response in the lung and LN.