2 Notch2 and B cell antigen receptor (BCR) signaling determine if transitional B cells become marginal zone B (MZB) or follicular B (FoB) cells in the spleen, but it isunknown how these pathways are related. We generated Taok3 equally important in B cell fate decisions [14][15][16] , and it was proposed that weak or strong BCR signals might render cells receptive or resistant to Notch instruction 17,18 . Yet how BCR repertoire or signaling controls Notch responsiveness is currently poorly understood.The Ste20 family kinases are serine-threonine kinases that participate in a variety of signaling pathways triggered by cellular stress 19,20 . The Tao kinase subfamily has three members in mammals (TAOK1, also known as proteins MAP3K16, PSK2 or MARKK; TAOK2 (MAP3K17, PSK1) and TAOK3 (MAP3K18, JIK or DPK)), whose function is largely unknown. Here, we generated Taok3 -/-mice and found that these mice lacked MZB cells, whereas FoB cells were intact. By carefully unraveling the molecular mechanism of this deficiency we have discovered how BCR signaling intersects with Notch signaling in immature transitional B cells undergoing positive selection. RESULTS 4 Taok3 -/-mice lack MZB cellsIn wild-type mice, the expression of mRNA for Taok3 was predominantly found in bone marrow and immune tissues like spleen, thymus and lymph nodes, but also in lung and gut (Fig. 1a). To gain insight into the biology of Taok3, we generated Taok3 -/-mice ( Supplementary Fig. 1a-e). Overall there was no difference in the cellularity of the various lymphoid organs in 6-12 week old mice (Supplementary Fig. 1f). In the spleen, there were no gross differences in the percentage of eosinophils, monocytes, natural killer (NK) cells and NKT cells between Taok3 -/-and wild-type mice, yet there was a consistent reduction in the amount of CD11c + dendritic cells in Taok3 -/-mice (Fig. 1b).There was a small yet consistent increase in the percentage of Ly6G + granulocytes in the spleen of Taok3 -/-mice. The distribution of CD3 + T cells and CD19 + B cells was comparable, yet there was a 25-30% reduction in the amount of CD8 + T cells (Fig. 1c). (Fig. 1d-f).Histological examination of the spleen revealed that the characteristic rim of IgM + CD1d + MZB cells separated from the IgM lo B cell follicles by the marginal sinus was absent in Taok3 -/-mice (Fig. 1g). Resident marginal zone metallophillic macrophages (MMM, expressing CD169) that line the marginal sinus were present and correctly localized in Taok3 -/-mice (Fig. 1h) (Fig. 1h). Collectively, Taok3 -/-mice lack MZB cells without compensatory alterations in other B cell subsets. Humoral immune response of Taok3 -/-miceThe baseline serum concentration of immunoglobulins (Ig) was comparable to wild-type, with a tendency for increased IgG3 in Taok3 -/-mice (Supplementary Fig 2) Fig. 1i and Supplementary Fig 2 ). The intact TNPspecific Ig response as not due to compensatory increase in recirculating MZB cells outside the spleen (data not shown). B1 B cells can also respond to TNP-Ficoll antigen, in the com...
ABSTACTThe type II secretion system (T2SS) is a multiprotein machinery spanning the diderm of gram-negative bacteria. T2SS contributes to the virulence of numerous gram-negative pathogens, including the multidrug resistant species Pseudomonas aeruginosa, Acinetobacter baumanii, Klebsiella pneumonia and Vibrio cholerae. Even though the T2SS has been studied extensively over the past three decades, our understanding of the molecular basis of its biogenesis and of its overall structure still remains unclear. Here we show that the core component of the inner membrane platform, the GspLM membrane protein complex, can be isolated as a dimer of dimers. Importantly, the complex is able to bind the T2SS ATPase, GspE, with high affinity. Finally, we have developed single domain VHH camelid antibodies (nanobodies) against the GspLM complex and have identified a nanobody that effectively prevents the cytoplasmic domain of GspL, GspLcyto, from binding to GspE. Our findings suggest that the T2SS ATPase is permanently associated with the inner membrane platform and that the GspELM complex should be considered as a key subassembly for the biogenesis of the T2SS apparatus.
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