Rapid activation of immune responses is necessary for antibacterial defense, but excessive immune activation can result in life-threatening septic shock. Understanding how these processes are balanced may provide novel therapeutic potential in treating inflammatory disease. Fc receptors are crucial for innate immune activation. However, the role of the putative Fc receptor for IgM, known as Toso/Faim3, has to this point been unclear. In this study, we generated Toso-deficient mice and used them to uncover a critical regulatory function of Toso in innate immune activation. Development of innate immune cells was intact in the absence of Toso, but Toso-deficient neutrophils exhibited more reactive oxygen species production and reduced phagocytosis of pathogens compared with controls. Cytokine production was also decreased in Toso −/− mice compared with WT animals, rendering them resistant to septic shock induced by lipopolysaccharide. However, Toso −/− mice also displayed limited cytokine production after infection with the bacterium Listeria monocytogenes that was correlated with elevated presence of Listeria throughout the body. Accordingly, Toso −/− mice succumbed to infections of L. monocytogenes , whereas WT mice successfully eliminated the infection. Taken together, our data reveal Toso to be a unique regulator of innate immune responses during bacterial infection and septic shock.

B cells are essential for antiviral immune defence because they produce neutralizing antibodies, present antigen and maintain the lymphoid architecture. Here we show that intrinsic signalling of CEACAM1 is essential for generating efficient B-cell responses. Although CEACAM1 exerts limited influence on the proliferation of B cells, expression of CEACAM1 induces survival of proliferating B cells via the BTK/Syk/NF-κB-axis. The absence of this signalling cascade in naive Ceacam1−/− mice limits the survival of B cells. During systemic infection with cytopathic vesicular stomatitis virus, Ceacam1−/− mice can barely induce neutralizing antibody responses and die early after infection. We find, therefore, that CEACAM1 is a crucial regulator of B-cell survival, influencing B-cell numbers and protective antiviral antibody responses.

Background/Aims: T-lymphocyte activation and function critically depends on Ca²⁺ signaling, which is regulated by store operated Ca²⁺ entry (SOCE). Human and mouse T lymphocytes express AMP activated kinase AMPKα1, which is rapidly activated following elevation of cytosolic Ca²⁺ concentration ([Ca²⁺]_i) by treatment of the cells with Ca²⁺ ionophore or following inhibition of endosomal Ca²⁺ ATPase with thapsigargin. AMPK is further activated by triggering of the T cell antigen receptor (TCR). The present study explored whether AMPK influences Ca²⁺ entry and Ca²⁺-sensitive regulation of T-lymphocyte function. Methods: T-lymphocytes were isolated and cultured from AMPKα1-deficient (ampk^-/-) mice and from their wildtype (ampk^+/+) littermates. The phenotype of the cells was analysed by flow cytometry, [Ca²⁺]_i estimated from Fura-2 fluorescence, SOCE from increase of [Ca²⁺]_i following thapsigargin treatment (1 µM), and cell function analysed by measuring cytokine secretion and western blotting. Results: Expression of surface markers in CD4⁺ and CD8⁺ T-cells were similar in ampk^-/- and ampk^+/+ T-lymphocyte blasts. Moreover, total STIM1 protein abundance was similar in ampk^-/- and ampk^+/+ T-lymphocyte blasts. However, Orai1 cell membrane protein abundance was significantly higher in ampk^-/- than in ampk^+/+ T-lymphocyte blasts. SOCE and increase of [Ca²⁺]_i following TCR activation by triggering TCR with anti-CD3 and cross-linking secondary antibody were both significantly more pronounced in ampk^-/- than in ampk^+/+ T-lymphocyte blasts. The difference of Ca²⁺ entry between ampk^-/- and ampk^+/+ T-lymphocytes was abrogated by Orai1 inhibitor 2-aminoethoxydiphenyl borate (2-APB, 50 µM). Proliferation of unstimulated ampk^-/- lymphocytes was higher than proliferation of ampk^+/+ T-lymphocytes, a difference reversed by Orai1 silencing. Conclusions: AMPK downregulates Orai1 and thus SOCE in T-lymphocytes and thus participates in negative feed-back regulation of cytosolic Ca²⁺ activity.