Positive selection of B cells expressing low densities of self-reactive BCRs.

B cell tolerance or autoimmunity is determined by selective events. Negative selection of self-reactive B cells is well documented and proven. In contrast, positive selection of conventional B cells is yet to be firmly established. Here, we demonstrate that developing self-reactive B cells are not always highly sensitive to the deletion mechanisms imposed by membrane-bound self-antigens. At low amounts, membrane-bound antigens allow survival of B cells bearing a single high affinity self-reactive B cell receptor (BCR). More importantly, we show that forced allelic inclusion modifies B cell fate; low quantities of self-antigen induce the selection and accumulation of increased numbers of self-reactive B cells with decreased expression of antigen-specific BCRs. By directly measuring antigen binding by intact B cells, we show that the low amounts of self-antigen select self-reactive B cells with a lower association constant. A fraction of these B cells is activated and secretes autoantibodies that form circulating immune complexes with self-antigen. These findings demonstrate that conventional B cells can undergo positive selection and that the fate of a self-reactive B cell depends on the quantity of self-antigen, the number of BCRs engaged, and on its overall antigen-binding avidity, rather than on the affinity of individual BCRs.

Monoallelic expression of the murine gene encoding Toll-like receptor 4.

Defects in the gene encoding Toll-like receptor 4 (Tlr4) result in impaired responses to lipopolysaccharide (LPS), rendering mice sensitive to infections by Gram-negative bacteria. C3H/HeJ mice have a codominant allele with a mutation in Tlr4, which results in an intermediate response to LPS in F1 mice from crosses of responder and C3H/HeJ mice. Here we show that this intermediate response to LPS is due to monoallelic expression of Tlr4. Allele usage is maintained during clonal expansion, a situation that resembles allelic exclusion. In contrast, Tlr4 is deleted on the recessive C57BL/10ScCr allele and all cells from F1 mice from crosses of responder and C57BL/10ScCr mice express TLR4 protein. Thus, Tlr4 is an autosomal gene whose expression is regulated similarly to that of genes on the X chromosome.

TLR4-dependent lipopolysaccharide-induced shedding of tumor necrosis factor receptors in mouse bone marrow granulocytes.

We reported previously that bone marrow granulocytes respond to small amounts of enterobacterial lipopolysaccharide (LPS) via a CD14-independent and TLR4-mediated mechanism by de novo expression of an inducible receptor (CD14) and by down-modulation of a constitutive receptor (L-selectin). In this report we address another effect of LPS: the down-regulation of receptors for tumor necrosis factor-alpha. In mouse bone marrow cells (BMC), this down-regulation is detectable soon (20 min) after exposure of the cells to low levels (0.5 ng/ml) of LPS. This temperature-dependent effect is rather selective for LPS and requires the presence of a conventional lipid A structure in the LPS molecule and a functional TLR4 molecule in the cells. The down-modulation, due to a shedding of the receptors, is blocked by p38 MAPK inhibitors, by a furin inhibitor, and by three metalloproteinase inhibitors (BB-3103, TIMP-2, and TIMP-3). In contrast, inhibitors of MEK, protein kinase C, cAMP-dependent protein kinase, and kinases of the Src family do not block the shedding. Analysis of BMC from mice lacking tumor necrosis factor receptor-1 (CD120a-/-) or tumor necrosis factor receptor-2 (CD120b-/-) indicates that the LPS-induced shedding is specific for CD120b. Thus, exposure of BMC to LPS triggers a rapid shedding of CD120b via a protein kinase C- and Src-independent pathway mediated by p38 MAPK, furin, and metalloproteinase. The additive effects of furin and metalloproteinase inhibitors suggest that these enzymes are involved in parallel shedding pathways.

Lipopolysaccharides from Legionella and Rhizobium stimulate mouse bone marrow granulocytes via Toll-like receptor 2.

Lipopolysaccharide (LPS) derived from enterobacteria elicit in several cell types cellular responses that are restricted in the use of Toll-like receptor 4 (TLR4) as the principal signal-transducing molecule. A tendency to consider enterobacterial LPS as a prototypic LPS led some authors to present this mechanism as a paradigm accounting for all LPSs in all cell types. However, the structural diversity of LPS does not allow such a general statement. By using LPSs from bacteria that do not belong to the Enterobacteriaceae, we show that in bone marrow cells (BMCs) the LPS of Rhizobium species Sin-1 and of three strains of Legionella pneumophila require TLR2 rather than TLR4 to elicit the expression of CD14. In addition, exposure of BMCs from TLR4-deficient (C3H/HeJ) mice to the lipid A fragment of the Bordetella pertussis LPS inhibits their activation by the Legionella lipid A. The data show selective action of different LPSs via different TLRs, and suggest that TLR2 can interact with many lipid A structures, leading to either agonistic or specific antagonistic effects.

Interaction of pulmonary surfactant protein C with CD14 and lipopolysaccharide.

In addition to their effects on alveolar surface tension, some components of lung surfactant also have immunological functions. We found recently that the hydrophobic lung surfactant protein SP-C specifically binds to the lipid A region of lipopolysaccharide (LPS). In this study, we show that SP-C also interacts with CD14. Four observations showed cross talk between the three molecules SP-C, LPS, and CD14. (i) Like LBP, SP-C allows the binding of a fluorescent LPS to cells expressing CD14 (the other surfactant components were ineffective). (ii) Recombinant radiolabeled CD14 and SP-C (or a synthetic analog of SP-C) interact in a dose-dependent manner. (iii) LPS blocks the binding of radiolabeled CD14 to SP-C-coated wells. (iv) SP-C enhances the binding of radiolabeled CD14 to LPS-coated wells. These results, obtained with native murine SP-C and with three synthetic analogs, suggest that LPS and CD14 interact with the same region of SP-C and that binding of SP-C modifies the conformation of CD14 or the accessibility of its LPS-binding site, allowing it to bind LPS. This ability of SP-C to interact with the pattern recognition molecule CD14 extends the possible immunological targets of SP-C to a large panel of microorganisms that can enter the airways.

Phase I study on sentinel lymph node mapping in colon cancer: a preliminary report.

BACKGROUND AND OBJECTIVES: Lymph node (LN) metastasis is one of the most significant prognostic factor in colorectal cancer. In fact, therapeutic decisions are based on LN status. However, multiple studies have reported on the limitations of the conventional pathological LN examination techniques, and therefore, the actual number of patients with LN positive colorectal cancer is probably underestimated. We assume that lymphatic tumor dissemination follows an orderly sequential route. We report here a simple and harmless coloration technique that was recently elaborated, and that allows us to identify the sentinel LN(s) (SLN) or first relay LNs in colorectal cancer patients. The main endpoint of this clinical trial is the feasibility of the technique. METHODS: Twenty patients treated by surgery for a colic cancer were admitted in this protocol. A subserosal peritumoral injection of lymphazurin 1% was performed 10 min before completing the colic resection. A pathologist immediately examined the specimens, harvested the colored SLN, and examined them by serial cuts (200 microm) with H&E staining, followed by immunohistochemical staining (AE1-AE3 cytokeratin markers), when serial sections were classified as cancer free. RESULTS: The preoperative identification of the SLN was impossible in at least 50 of the cases, however, SLNs were identified by the pathologist in 90% of cases. In two patients (10%) SLN was never identified. The average number of SLN was 3.9. Immunohistochemical analysis of the SLN has potentially changed the initial staging (from Dukes B to Dukes C) for 5 of the 20 patients (25%). On the other hand, there was one patient (5%) with hepatic metastasis from adenocarcinoma for whom SLN pathology was negative for metastasis (skip metastasis). CONCLUSIONS: SLN biopsy is readily feasible with identification of SLN in at least 90% of patients with colorectal cancers. Our results indicate that 45% of patients initially staged as Dukes B had tumor cells identified in their SLN when these were subjected to our protocol. This represented a 25% upgrading rate when our complete study population is considered. However, controversy persist about the clinical significance and metastatic potential of these often very small clusters of tumor cells.