Rac1 activation inhibits E-cadherin-mediated adherens junctions via binding to IQGAP1 in pancreatic carcinoma cells.

BACKGROUND: Monomeric GTPases of the Rho family control a variety of cellular functions including actin cytoskeleton organisation, cell migration and cell adhesion. Defects in these regulatory processes are involved in tumour progression and metastasis. The development of metastatic carcinoma is accompanied by deregulation of adherens junctions, which are composed of E-cadherin/beta- and alpha-catenin complexes. RESULTS: Here, we show that the activity of the monomeric GTPase Rac1 contributes to inhibition of E-cadherin-mediated cell-cell adhesion in pancreatic carcinoma cells. Stable expression of constitutively active Rac1(V12) reduced the amount of E-cadherin on protein level in PANC-1 pancreatic carcinoma cells, whereas expression of dominant negative Rac1(N17) resulted in an increased amount of E-cadherin. Extraction of proteins associated with the actin cytoskeleton as well as coimmunoprecipitation analyses demonstrated markedly decreased amounts of E-cadherin/catenin complexes in Rac1(V12)-expressing cells, but increased amounts of functional E-cadherin/catenin complexes in cells expressing Rac1(N17). Cell aggregation and migration assays revealed, that cells containing less E-cadherin due to expression of Rac1(V12), exhibited reduced cell-cell adhesion and increased cell motility. The Rac/Cdc42 effector protein IQGAP1 has been implicated in regulating cell-cell adhesion. Coimmunoprecipitation studies showed a decrease in the association between IQGAP1 and beta-catenin in Rac1(V12)-expressing PANC-1 cells and an association of IQGAP1 with Rac1(V12). Elevated association of IQGAP1 with the E-cadherin adhesion complex via beta-catenin correlated with increased intercellular adhesion of PANC-1 cells. CONCLUSION: These results indicate that active Rac1 destabilises E-cadherin-mediated cell-cell adhesion in pancreatic carcinoma cells by interacting with IQGAP1 which is associated with a disassembly of E-cadherin-mediated adherens junctions. Inhibition of Rac1 activity induced increased E-cadherin-mediated cellular adhesion.

Salmonella type III-mediated heterologous antigen delivery: a versatile oral vaccination strategy to induce cellular immunity against infectious agents and tumors.

Salmonella type III secretion system (T3SS)-mediated translocation can be used for efficient delivery of heterologous antigens to the cytosol of antigen-presenting cells leading to prominent CD8 T-cell priming in orally immunized mice. The time point and duration of hybrid protein translocation during the Salmonella infection cycle can be modulated by employing various type III carrier molecules. The p60 protein of Listeria monocytogenes was used as model antigen to construct chimeric SspH2/p60. SspH2 is a "Salmonella pathogenicity island 2" (SPI2) protein that is known to be translocated by Salmonella during intracellular survival and replication in macrophages. This SPI2 carrier molecule is sufficient to induce a concomitant p60-specific CD4 and CD8 T-cell response in Salmonella-vaccinated mice. Moreover, T3SS-mediated antigen delivery results in an efficient priming of central and effector memory CD8 T cells in spleens of these animals. This vaccination strategy can also be employed to efficiently protect mice from an aggressive fibrosarcoma transfected with p60 in a prophylactic setting.

Pre-existing anti-Salmonella vector immunity prevents the development of protective antigen-specific CD8 T-cell frequencies against murine listeriosis.

Our laboratory has focused its research on the use of the type III secretion system of Salmonella enterica serovar Typhimurium to translocate heterologous antigens directly into the cytosol of antigen-presenting cells. We have previously reported that the single oral immunization of mice with a recombinant Salmonella aroA/sptP mutant strain expressing the translocated Yersinia outer protein E fused to the immunodominant antigen p60 from Listeria monocytogenes in a type III-mediated fashion results in the efficient induction of p60-specific CD8 T cells and confers protection against a lethal Listeria challenge infection. In the present study, we determined whether pre-existing anti-Salmonella vector immunity influences the induction of p60-specific CD8 T cells and modulates protective immunity against listeriosis after oral vaccination with recombinant Salmonella. After single oral immunization, the Salmonella aroA/sptP double mutant strain was found to colonize spleens of mice for 21days. In contrast, the period of colonization was significantly shortened to 6days due to anti-Salmonella vector immunity after second oral immunization. The latter scenario led to the induction of low-level frequencies of antigen-specific CD8 T cells. Compared to the significantly higher numbers of p60-specific T lymphocytes elicited after single oral immunization, the low amount of Listeria-specific CD8 T cells did not confer protection against listeriosis.

Prophylactic anti-tumor immunity against a murine fibrosarcoma triggered by the Salmonella type III secretion system.

The potential of an attenuated Salmonella enterica serovar Typhimurium strain as a prophylactic anti-tumor vaccine against the murine fibrosarcoma WEHI 164 was evaluated. Tumor cells were transfected with the DNA sequence encoding the MHC class I-restricted peptide p60(217-225) from Listeria monocytogenes. BALB/c mice received a single orogastric immunization with Salmonella that translocates a chimeric p60 protein via its type III secretion system. Mice were subsequently challenged subcutaneously with p60(217-225)-expressing WEHI cells. In vivo protection studies revealed that 80% of these mice remained free of the fibrosarcoma after challenge, whereas all animals of the non-vaccinated control group did develop tumor growth. In further experiments, the distribution of tetramer-positive p60(217-225)-specific effector and memory CD8 T cells after Salmonella-based immunization and tumor application was analyzed. Costaining with CD62L and CD127 revealed a predominance of p60-specific central memory and effector memory CD8 T cells in spleens, whereas in blood samples the majority of p60-specific lymphocytes belonged to effector and effector memory CD8 T cell subsets. This is the first report demonstrating that a bacterial type III secretion system can be used for heterologous antigen delivery to induce cytotoxic effector and memory CD8 T cell responses resulting in an efficient prevention of tumor growth.

Rapid clearance of a recombinant Salmonella vaccine carrier prevents enhanced antigen-specific CD8 T-cell responses after oral boost immunizations.

The type III secretion system of Salmonella enterica serovar Typhimurium can be used to target heterologous antigens directly into the cytosol of antigen-presenting cells. Our laboratory has previously reported that the single oral immunization of mice with a recombinant Salmonella strain expressing the translocated Yersinia outer protein E fused to the immunodominant antigen p60 from Listeria monocytogenes results in the efficient induction of p60-specific CD8 T cells and confers protection against a lethal wild-type Listeria challenge infection. In the present study, we investigated whether these antigen-specific cytotoxic T lymphocytes induced by the prime immunization contribute to a more rapid clearance of the vaccine carrier after subsequent boost immunizations and whether oral boost immunizations lead to an augmented p60-specific CD8 T-cell response. We found that the ability of recombinant Salmonella strains to colonize the intestine, mesenteric lymph nodes, and spleen was markedly impaired after boost immunizations but that this effect was independent of existing CD8 T cells reactive with p60(217-225). A significant elevation of antigen-specific CD8 T cells could not be detected by enzyme-linked immunospot assay after the second or the third oral immunization, possibly due to the rapid clearance of the bacterial vaccine carrier from lymphatic organs.

Salmonella pathogenicity island 2-mediated overexpression of chimeric SspH2 proteins for simultaneous induction of antigen-specific CD4 and CD8 T cells.

Salmonella enterica serovar Typhimurium employs two different type III secretion systems (TTSS) encoded within Salmonella pathogenicity islands 1 and 2 (SPI1 and SPI2) for targeting of effector proteins into the cytosol of eukaryotic cells during different stages of the infection cycle. The SPI1 TTSS translocates virulence factors across the plasma membrane when the bacterium initially contacts the host cell. In contrast, the SPI2 TTSS functions to translocate proteins across the membrane of the Salmonella-containing vacuole and promotes intracellular survival and replication. The aim of the present study was to directly compare the potentials of SPI1 and SPI2 type III effector proteins to act as carrier molecules for a heterologous antigen. The p60 protein of Listeria monocytogenes was used as a model antigen to construct chimeric SopE2 (SPI1), SifA (SPI2), and SspH2 (SPI2) proteins. SPI1- and SPI2-dependent up- and down-regulation of hybrid gene expression led to sequential translocation of p60 fusion proteins into the cytosol of Salmonella-infected macrophages. Mice orally immunized with recombinant Salmonella strains expressing these hybrid proteins revealed comparable numbers of p60-specific CD8 T cells. However, only overexpression of translocated SspH2/p60 from a medium-copy-number vector induced simultaneous antigen-specific CD4 and CD8 T-cell responses, suggesting that SspH2 is an attractive carrier molecule for foreign-protein delivery.