{
  "abstract": "Background The gut microbiota has profound influence on the host immune system with implications in various pathological states. In melanoma patients the gut microbiota composition is associated with the efficacy of anti-PD-1 immunotherapy, 1 and fecal microbiota transplant (FMT) from responder patients can in some cases improve efficacy in non-responders.2 Understanding the mechanisms by which the gut microbiota can influence immunotherapy outcomes could further improve therapeutic strategies.Methods Stool from anti-PD-1 responder and non-responder melanoma patients was transferred to germ-free mice, which were then bred to establish stable colonies. In this model, offspring mice acquired microbiota naturally from parental mice, and the microbiota composition remained similar to the original patient donor over multiple generations. Importantly, responsiveness versus non-responsiveness to PD-1/PD-L1 blockade was maintained generationally. To determine the microbiota-conferred effects on gene expression programing of immune cells in the tumor microenvironment, CD45 + tumor-infiltrating cells were analyzed using single-cell RNA sequencing (scRNAseq). Melanoma patient biopsies were analyzed using bulk RNA sequencing to determine if similar gene expression patterns were associated with anti-PD-1 efficacy in the clinic. These gene expression patterns were further validated in a published dataset of tumor bulk RNA sequencing from a clinical trial utilizing responder FMT as a therapeutic approach to improve anti-PD-1 efficacy.2 To determine whether observed shifts in myeloid phenotype might be observed throughout the host, splenocytes from non-tumor-bearing mice were also analyzed by scRNAseqResults In the tumor microenvironment, scRNAseq revealed marked differences in myeloid cell genes in mice colonized with responder versus non-responder microbiota. In particular, a pro-inflammatory phenotype in both the macrophage and neutrophil lineages was seen in responder mice as opposed to immunosuppressive programming in non-responders. In patient melanoma samples, a higher ratio of pro-inflammatory to immunosuppressive macrophage-associated gene expression correlated with improved anti-PD-1 efficacy. In patients treated by FMT, upregulation of pro-inflammatory to immunosuppressive macrophage-associated genes was observed post-treatment in responding patients. Surprisingly, in non-tumor-bearing mice, the shifts in myeloid transcriptional signatures were already observed. Genes associated with interferon signaling were upregulated in monocytes/macrophages in responder-microbiota mice, whereas cell division genes were enriched in non-responder microbiota mice.Conclusions These results suggest that the gut microbiota might impact the efficacy of PD-1/PD-L1 blockade therapy by modulation of myeloid cell homeostatic signaling system-wide. Microbiome interventions might work, in part, by promoting favorable myeloid cell differentiation states.References Matson V, Fessler J, Bao R, et al. The commensal microbiome is associated with anti-PD-1 efficacy in metastatic melanoma patients. Science. 2018;359:104–108.Baruch EN, Youngster I, Ben-Betzalel G, et al. Fecal microbiota transplant promotes response in immunotherapy-refractory melanoma patients. Science. 2020;371:602–608.Ethics Approval This study was approved by The University of Chicago Research Administration, approval number FP069810. Participants gave informed consent before taking part.",
  "authors": [
    {
      "affiliations": [
        "The University of Chicago, Chicago, IL, USA"
      ],
      "name": "Vyara Matson"
    },
    {
      "affiliations": [
        "Stanford University, Palo Alto, CA, USA"
      ],
      "name": "Jessica L Fessler"
    },
    {
      "affiliations": [
        "The University of Chicago, Chicago, IL, USA"
      ],
      "name": "Jason Shapiro"
    },
    {
      "affiliations": [
        "The University of Chicago, Chicago, IL, USA"
      ],
      "name": "Ruxandra Tonea"
    },
    {
      "affiliations": [
        "The University of Chicago, Chicago, IL, USA"
      ],
      "name": "Thomas F Gajewski"
    }
  ],
  "title": "1139 The gut microbiota shapes the homeostatic programming of myeloid cells systemically to impact PD-1/PD-L1 blockade therapy",
  "uid": "68b960b5-2fa4-5f3c-a82f-c6ce37505ccc"
}