The riboflavin (vitamin B 2 ) biosynthetic genes in Bacillus subtilis are transcribed simultaneously from the riboflavin promoter (P rib ). The 5'-end of the nascent rib-mRNA carries a flavin mononucleotide (FMN) binding riboswitch, which regulates gene expression. The antibiotic roseoflavin from Streptomyces davawensis is a naturally occurring riboflavin analog, its mechanism of action is largely unknown. A recombinant B. subtilis strain carrying a copy of P rib -RFN fused to a promoterless lacZ reporter gene in the chromosomal amyE locus was grown in a minimal medium. Upon addition of roseoflavin to the growth medium the apparent LacZ activity in this strain was not significantly reduced. Similar experiments carried out on recombinant B. subtilis strains oversynthesizing the flavin transporters RibU (B. subtilis) or RibM (S. davawensis) produced still other results. In these strains, roseoflavin (as well as riboflavin) repressed LacZ synthesis indicating that the RFN riboswitch is a target for roseoflavin (or roseoflavin mononucleotide), which may at least in part explain its antibiotic activity.

Purpose: The recent success of anti-PD1 antibody in metastatic colorectal cancer (CRC) patients with microsatellite instability (MSI), known to be associated with an upregulated Th1/Tc1 gene signature, provides new promising therapeutic strategies. However, the partial objective response highlights a crucial need for relevant, easily evaluable, predictive biomarkers. Here we explore whether in situ assessment of Tbet+ tumor infiltrating lymphocytes (TILs) reflects a pre-existing functional antitumor Th1/Tc1/IFNγ response, in relation with clinicopathological features, microsatellite status and expression of immunoregulatory molecules (PD1, PDL1, IDO-1). Methodology: In two independent cohorts of CRC (retrospective n = 80; prospective n = 27) we assessed TILs density (CD3, Tbet, PD1) and expression profile of PDL1 and IDO-1 by immunohistochemistry/image analysis. Furthermore, the prospective cohort allowed to perform ex vivo CRC explant cultures and measure by Elisa the IFNγ response, at baseline and upon anti-PD1 treatment. Results: The density of Tbet+ TILs was significantly higher in MSI CRC, especially in the medullary subtype but also in a subgroup of MSS (microsatellite stable), and positively correlated with PD1 and PDL1 expression, but not with IDO-1. Finally, a high number of Tbet+ TILs was associated with a favorable overall survival. These Tbet+ TILs were functional as their density positively correlated with basal IFNγ levels. In addition, the combined score of Tbet+ PD1+ TILs coupled with IDO-1 expression predicted the magnitude of the IFNγ response upon anti-PD1. Conclusion: Altogether, immunohistochemical quantification of Tbet+ TILs is a reliable and accurate tool to recapitulate a preexisting Th1/Tc1/IFNγ antitumor response that can be reinvigorated by anti-PD1 treatment.

590 Background: PD-1/PD-L1 blockade showed therapeutic efficacy in only microsatellite (MSI) colorectal carcinomas (CRC), however, the profile of PD-L1 and PD-1 expression in CRC is only partially described. Methods: We thus analyzed on FFPE whole-tissue sections of 80 CRC, the expression profile of PD-L1 by tumor and/or immune cells by immunohistochemistry (clone E1L3N) depending on the MSI status and the histological subtype, and correlated to the density of PD-1+ and Tbet+ (able to secrete IFNg known to induce PD-L1) tumor-infiltrating lymphocytes (TIL). Results: 78% of MSI CRC (32/41) overexpressed PD-L1 either by tumor or immune cells versus 46% of MSS CRC (18/39) (p 0.005). This overexpression was heterogeneous within the same tumor in most of cases. Among MSI CRC, PD-L1 was preferentially overexpressed in medullary carcinomas (MC, 19/21, 90%) compared with 65% (13/20) in non-medullary adenocarcinomas (p 0.06). PD-L1 expression by tumor cells was only observed in MSI CRC (19/41, 46% with PD-L1 expression in more than 5% of tumor cells – score 1), and preferentially in MC (57% vs 5% in no medullary adenocarcinomas, with PD-L1 expression in more than 50% of tumor cells – score 3, p 0.0005). Conversely, PD-L1 expression by immune cells was observed in MSI CRC (23/41, 56% with PD-L1 expression by more than 5 sheets of 50 positive cells) but also in MSS CRC (18/39, 43%) (p 0.5). The density of PD-1+ cells was significantly correlated to the PD-L1 expression, as well as the density of Tbet+ TIL. Conclusions: PD-L1 expression is 1) heterogeneous in CRC, among CRC but also within the same tumor, 2) preferentially observed in MSI CRC (78%), especially in MC (90%), where PD-L1 is expressed by tumor cells, 3) correlated with the density of PD-1+ or T-bet+ TIL, and 4) observed in a significant proportion of MSS CRC (46%) by immune cells only. From a clinical point of view, PD-L1 expression has to be determined at best in full tissue section and besides its preferential expression in MSI CRC, its significant frequency and expression profile (only by immune cells) in MSS CRC should be taken into account in the future clinical trials testing the efficacy of anti-PD-1/PD-L1 antibodies.

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e14599 Background: PD1/PDL1 blockade showed therapeutic efficacy in only microsatellite (MSI) colorectal carcinomas (CRC), however, the profile of PDL1 and PD1 expression in CRC is only partially described. Methods: We thus analyzed on FFPE whole-tissue sections of 80 CRC, the expression profile of PDL1 by tumor and/or immune cells by immunohistochemistry (clone E1L3N) depending on the MSI status and the histological subtype, and correlated to the density of PD1+ and Tbet+ (able to secrete IFNg known to induce PDL1) tumor-infiltrating lymphocytes (TIL). Results: 78% of MSI CRC (32/41) overexpressed PDL1 either by tumor or immune cells versus 46% of MSS CRC (18/39) (p 0.005). This overexpression was heterogeneous within the same tumor in most of cases. Among MSI CRC, PDL1 was preferentially overexpressed in medullary carcinomas (MC, 19/21, 90%) compared with 65% (13/20) in non medullary adenocarcinomas (p 0.06). PDL1 expression by tumor cells was only observed in MSI CRC (19/41, 46% with PDL1 expression in more than 5% of tumor cells – score 1), and preferentially in MC (57% vs 5% in no medullary adenocarcinomas, with PDL1 expression in more than 50% of tumor cells – score 3, p 0.0005). Conversely, PDL1 expression by immune cells was observed in MSI CRC (23/41, 56% with PDL1 expression by more than 5 sheets of 50 positive cells) but also in MSS CRC (18/39, 43%) (p 0.5). The density of PD1+ cells was significantly correlated to the PDL1 expression, as well as the density of Tbet+ TIL. Conclusions: PDL1 expression is 1) heterogeneous in CRC, among CRC but also within the same tumor, 2) preferentially observed in MSI CRC (78%), especially in MC (90%), where PDL1 is expressed by tumor cells, 3) correlated with the density of PD1+ or Tbet+ TIL, and 4) observed in a significant proportion of MSS CRC (46%) by immune cells only. From a clinical point of view, PDL1 expression has to be determined at best in full tissue section and besides its preferential expression in MSI CRC, its significant frequency and expression profil (only by immune cells) in MSS CRC should be taken into account in the future clinical trials testing the efficacy of anti-PD1/PDL1 antibodies.