Programmed cell death 1 inhibits inflammatory helper T-cell development through controlling the innate immune response.

Programmed cell death 1 (PD-1) is an inhibitory coreceptor on immune cells and is essential for self-tolerance because mice genetically lacking PD-1 (PD-1(-/-)) develop spontaneous autoimmune diseases. PD-1(-/-) mice are also susceptible to severe experimental autoimmune encephalomyelitis (EAE), characterized by a massive production of effector/memory T cells against myelin autoantigen, the mechanism of which is not fully understood. We found that an increased primary response of PD-1(-/-) mice to heat-killed mycobacteria (HKMTB), an adjuvant for EAE, contributed to the enhanced production of T-helper 17 (Th17) cells. Splenocytes from HKMTB-immunized, lymphocyte-deficient PD-1(-/-) recombination activating gene (RAG)2(-/-) mice were found to drive antigen-specific Th17 cell differentiation more efficiently than splenocytes from HKMTB-immunized PD-1(+/+) RAG2(-/-) mice. This result suggested PD-1's involvement in the regulation of innate immune responses. Mice reconstituted with PD-1(-/-) RAG2(-/-) bone marrow and PD-1(+/+) CD4(+) T cells developed more severe EAE compared with the ones reconstituted with PD-1(+/+) RAG2(-/-) bone marrow and PD-1(+/+) CD4(+) T cells. We found that upon recognition of HKMTB, CD11b(+) macrophages from PD-1(-/-) mice produced very high levels of IL-6, which helped promote naive CD4(+) T-cell differentiation into IL-17-producing cells. We propose a model in which PD-1 negatively regulates antimycobacterial responses by suppressing innate immune cells, which in turn prevents autoreactive T-cell priming and differentiation to inflammatory effector T cells.

PECAM-1 ligation negatively regulates TLR4 signaling in macrophages.

Uncontrolled TLR4 signaling may induce excessive production of proinflammatory cytokines and lead to harmful inflammation; therefore, negative regulation of TLR4 signaling attracts much attention now. PECAM-1, a member of Ig-ITIM family, can mediate inhibitory signals in T cells and B cells. However, the role and the mechanisms of PECAM-1 in the regulation of TLR4-mediated LPS response in macrophages remain unclear. In this study, we demonstrate that PECAM-1 ligation with CD38-Fc fusion protein negatively regulates LPS-induced proinflammatory cytokine TNF-alpha, IL-6, and IFN-beta production by inhibiting JNK, NF-kappaB, and IFN regulatory factor 3 activation in macrophages. In addition, PECAM-1 ligation-recruited Src homology region 2 domain-containing phosphatase 1 (SHP-1) and Src homology region 2 domain-containing phosphatase 2 (SHP-2) may be involved in the inhibitory effect of PECAM-1 on TLR4 signaling. Consistently, silencing of PECAM-1 enhances the macrophage response to LPS stimulation. Taken together with the data that PECAM-1 is constitutively expressed in macrophages and its expression is up-regulated by LPS stimulation, PECAM-1 might function as a feedback negative regulator of LPS inflammatory response in macrophages. This study may provide a potential target for intervention of inflammatory diseases.