Crimson Publishers Publish With Us Reprints e-Books Video articles

Full Text

Advances in Complementary & Alternative medicine

Gut Microbiome and Mechanisms of Primary and Acquired Resistance To PD-1/PD-L1 Blockade

Maria Kuman*

Holistic Research Institute, USA

*Corresponding author:Maria Kuman, PhD, Holistic Research Institute, Knoxville, Tennessee, 37923, USA

Submission: August 19, 2021;Published: September 21, 2021

DOI: 10.31031/ACAM.2020.06.000645

ISSN: 2637-7802
Volume 6 Issue 4


PD-1/PD-L1 blockade is a promising immunotherapy, which provides a new method for the treatment of a variety of tumors and has significant clinical efficacy. However, most patients are not initially sensitive to these therapies, known as primary resistance. Unfortunately, some patients develop acquired resistance even after an initial response to PD-1/PD-L1 blockade. In fact, the mechanisms of primary and acquired resistance are not fully and clearly understood. Recently, the role of gut microbiome has also become a hot topic of research. From the above aspects, this article will discuss the related mechanisms and new strategies to improve the curative effect.

Keywords: Immune checkpoint blockade therapy; Immunotherapy resistance; Microbiome


Immunotherapy has certainly opened up a promising new field of research in cancer treatment. A variety of malignant tumors, such as metastatic melanoma, non-small-cell lung cancer (NSCLC), head and neck squamous cell cancer, Hodgkin’s lymphoma, renal cell carcinoma, urothelial carcinoma, Merkel cell carcinoma, gastric carcinoma, and hepatocellular carcinomas [1-15]. Frustratingly, the efficacy of monotherapy for PD-1/PD-L1 blocking is generally less than 40% in most malignancies [5,16]. Approximately 60% of patients with melanoma, the most sensitive type of tumor to immunotherapy, also show primary resistance to PD-1/PD-L1 blocking therapy [16]. The initial effect of treatment would wear off over time, resulting in continued deterioration or recurrence [17].

Microbial imbalance plays a significant role in cancer as well. Certain bacteria and their metabolites contribute greatly to the restoration of natural beneficial microbiome [18]. In brief, this review will focus on the complex and dynamic mechanisms responsible for resistance to immunotherapy via PD-1/PD-L1 blockade and the gut microbiome.

Resistance Mechanisms

Lack of tumor immunogenicity

Expression and presentation of antigens and neoantigens are the core of T cells’ ability to recognize tumors and participate in TCR. T cells lack the capacity to recognize tumors due to absence of tumor antigens, which directly gives rise to the inability of host CD8+ T cells to localize to tumors [19]. Therefore, anti-PD-1 /PD-L1 therapy is more effective in tumors with high mutation load and increased neoantigen expression, including melanoma, NSCLC, and microsatellite unstable tumors [5,20-22]. In contrast, tumors with fewer somatic mutations, like pancreatic and prostate cancers, are generally less sensitive to PD-1/PD-L1 blocking [21,23].
In addition, Cancer cells would silence or alter the expression of antigen-presenting machinery, beta-2-microglobulin (β2M) or MHC molecules, thereby preventing antigen processing and presentation to the cell surface [24,25]. Excessive cell proliferation and DNA damage caused by chronic inflammation can induce CRP to increase the mutational burden of local tumors, reducing the resistance of tumors to PD-1 therapy [26].

Tumor microenvironment and T cell exclusion

A phenomenon called T cell exclusion may happen in metastatic melanoma, bladder transitional cell carcinoma etc., which is caused by primary mutational events within the tumor. T cells tracking to the tumor microenvironment are inhibited without influencing antigen expression or presentation. Some abnormal cell signal transduction pathways, such as PI3K/ AKT pathway, WNT/β -catenin pathway, mitogen activated protein kinase (MAPK) pathway and NF-κ B pathway, are essential to explain this phenomenon [27].
However, chronic infection and cancer expose CD8+ T cells to continuous antigenic stimulation, on which PD-1 expression gradually occurs. Stimulation of PD-1 can result in another state of T cell dysfunction, called T cell exhaustion. The depleted T cells possess poor effector function, suppressed receptor expression and abnormal transcriptional status. PD-1 blockade can reactivate these hypofunctional “exhausted” CD8+ T cells (TEX) and restore their function to fight tumors [28].
Tumeh et al. found that preexisting CD8+ T cells are the prerequisite for tumor regression after PD-1/PD-L1 blocking therapy in metastatic melanoma, suggesting that tumor-infiltrating lymphocytes are important components of the response to anti- PD-1 therapy [29]. In fact, in addition to tumor cells, there are many components in TME that may be related to primary or acquired drug resistance, including myeloid derived suppressor cells (MDSCs), Tregs, TAMs, IDO and so on. They are closely related to tumor cells, protecting tumor cells from detection and destruction through immune monitoring [30]. Apart from immune regulation factors, co-enrichment of a set of 26 transcriptomic markers (known as IPRES signatures) is also involved in primary resistance to PD-1/ PD-L1 [17, 31].

Tumor cell resistance to interferon

CD8+ T cells that have identified and participated in appropriate tumor antigens can produce IFN-γ, thereby increasing MHC expression/antigen presentation, attracting more T cells into tumors, and directly inducing anti-proliferation and apoptosis of cancer cells [32]. The success of any T-cell-based immunotherapy such as PD-1/PD-L1 blockade is dependent on the response of interferon to tumors. Although mutations within interferon signaling elements have been described in the setting of primary resistance to treatment, chances are that these mutations occur after treatment has begun [27]. Besides reflecting the dynamic response of IFN-γ, PD-L1 expression can also be expressed constitutionally under certain circumstances. Patients diagnosed with NSCLC with EGFR mutations and ALK rearrangement are extremely insensitive to PD-1/PD-L1 inhibitors [33-35].

Gut microbiome

Microbial imbalance, such as reduced bacterial population and changes in their species composition, can contribute to tumorigenesis or immunotherapy failure. By promoting nutrient absorption, metabolism and immune development, gut microbes can promote tissue growth and differentiation, which increases the risk of cancer greatly [36]. Research found that mutated p53 drives tumor inhibition by disrupting the WNT pathway, through preventing TCF4 from binding to chromatin. Frustratingly, this inhibitory effect can be eliminated by gut microbiome completely [37]. However, on the other hand, gut microbiome is a beneficial potential regulator, which tightly connects intestinal cells, reduces intestinal permeability, and inhibits carcinogenicity to a certain extent [38]. Adoptive metastasis of tumor-specific T cells induces translocation of intestinal flora from the lumen to mesenteric lymph nodes in mice, demonstrating for the first time the positive effect of intestinal flora in cancer treatment [39]. Gut microbiome can reverse the resistance to immunity via increasing production of cytokines, enhancing the activation of DCs, decreasing peripherally derived Tregs, inducing the overexpression of chemokines and so on [38]. Shotgun metagenomes from the same sample revealed that patients with different responses differed in the abundance of pathways related to nucleoside and nucleotide biosynthesis, lipid biosynthesis, glucose metabolism, and fermentative shortchain fatty acids (SCFAs). The presence of gut bacteria capable of producing SCFA, like Eubacterium, Lactobacillus, and Streptococcus, significantly enhances patients’ response to anti-PD-1/PD-L1 across different types of GI cancer [40]. Actually, the adjuvant role of gut microbiome in the immune checkpoint inhibitor treatment of advanced melanoma, including NSCLC, RCC, and urothelial carcinoma has attracted considerable attention [41,42]. In a mouse model, Daillere and colleagues identified that Enterococcus hirae and Barnesiella intestinihominis are key steps in the anti-tumor and immunomodulatory properties of cyclophosphamide [43,44]. Grifn et al. [18] suggested that oral administration of Enterococcal bacteria, including E. hirae, E. Durans, and E. Mundtii, is an effective method of anti-PD-L1 immunotherapy. Wang et al. [45] believed that A. muciniphila could regulate intestinal homeostasis and reshape the immune environment. An experiment by Zheng et al. [46] showed that fecal samples from patients who responded to immunotherapy were more likely to be found with high abundance and a variety of bacterial genes. Take a typical example is that almost all fecal samples from patients who responded to treatment with camrelizumab were able to find approximately 20 species, such as Akkermansia Muciniphila and Ruminococcaceae SPP.
In fact, the pattern of interaction between gut microbes and the host immune system still needs to be further identified. Three possible ideas have been proposed:
(1) via T cell responses induced by microbial antigens,
(2) via the involvement of pattern recognition receptors, and
(3) via small molecules produced by microbial metabolism [47]
Meanwhile, probiotics, as a kind of microbe promoting human health, can inhibit the proliferation of tumor cells through regulating gut microbiome [48] and immune regulation [49,50].
The remarkably decreased expression of the inflammatory cytokine IL-17 in tumors was closely affiliated with the inhibition of Th17 cell population and Th17 cell infiltration in intestinal and peripheral circulation. Probiotics supplementation also helped upregulate the expression of the anti-inflammatory cytokines IL-10, IL-13, and IL-27 [51].


Although PD-1/PD-L1 blocking immunotherapy has shown great promise in the treatment of various advanced cancers, there are still many difficulties, and there is still a long way to go to improve the efficacy of PD-1/PD-L1 blocking therapy comprehensively and individually. Given the complexity of the interaction between cancer and the immune system, multi-drug combination therapy may be a better option than single-drug therapy. For example, the coadministration of PD-1/PD-L1 blockade with tumor necrosis factor inhibitors [52,53], metformin [54], anti-VEGF drugs [55], or other immune checkpoint inhibitors (such as CXCR4[56]) has been verified to amplify anti-tumor efficacy and reduce toxicity. With the further study of gut microbiome and immunotherapy biomarkers, researchers will open more new research fields and directions in tumor immunotherapy. In all, with mechanisms responsible for resistance continuing to be characterized, therapies can be personalized and change in real time according to patients’ responses and condition, effectively overcoming relapse. More comprehensive and personalized treatment strategies are bound to contribute to more and more patients.


  1. Abril-Rodriguez G and Ribas A (2017) SnapShot: Immune Checkpoint Inhibitors. Cancer Cell 31(16): 848-848 e1.
  2. Hamid O, Robert C, Daud A, Hodi FS, Hwu WJ, et al. (2013) Safety and tumor responses with lambrolizumab (anti-PD-1) in melanoma. N Engl J Med 369(2): 134-144.
  3. Robert C, Ribas A, Wolchok JD, Hodi FS, Hamid O, et al. (2014) Anti-programmed-death-receptor-1 treatment with pembrolizumab in ipilimumab-refractory advanced melanoma: a randomised dose-comparison cohort of a phase 1 trial. Lancet 384(9948): 1109-1117.
  4. Sundar R, Cho BC, Brahmer JR, Soo RA (2015) Nivolumab in NSCLC: latest evidence and clinical potential. Ther Adv Med Oncol 7(2): 85-96.
  5. Topalian SL, Hodi FS, Brahmer JR, Gettinger SN, Smith DC, et al. (2012) Safety, activity, and immune correlates of anti-PD-1 antibody in cancer. N Engl J Med 366(26): 2443-2454.
  6. Brahmer JR, Tykodi SS, Chow LQ, Hwu WJ, Topalian SL, et al. (2012) Safety and activity of anti-PD-L1 antibody in patients with advanced cancer. N Engl J Med 366(26): 2455-2465.
  7. Armand P, Nagler A, Weller EA, Devine SM, Avigan DE, et al. (2013) Disabling immune tolerance by programmed death-1 blockade with pidilizumab after autologous hematopoietic stem-cell transplantation for diffuse large B-cell lymphoma: results of an international phase II trial. J Clin Oncol 31(33): 4199-4206.
  8. Herbst RS, Soria JC, Kowanetz M, Fine GD, Hamid O, et al. (2014) Predictive correlates of response to the anti-PD-L1 antibody MPDL3280A in cancer patients. Nature 515(7528): 563-567.
  9. Powles T, Eder JP, Fine GD, Braiteh FS, Loriot Y, et al. (2014) MPDL3280A (anti-PD-L1) treatment leads to clinical activity in metastatic bladder cancer. Nature 515(7528): 558-562.
  10. Robert C, Long GV, Brady B, Dutriaux C, Maio M, et al. (2015) Nivolumab in previously untreated melanoma without BRAF mutation. N Engl J Med 372(4): 320-330.
  11. Ansell SM, Lesokhin AM, Borrello I, Halwani A, Scott EC, et al. (2015) PD-1 blockade with nivolumab in relapsed or refractory Hodgkin's lymphoma. N Engl J Med 372(4): 311-319.
  12. Robert C, Schachter J, Long GV, Arance A, Grob JJ, et al. (2015) Pembrolizumab versus Ipilimumab in Advanced Melanoma. N Engl J Med 372(26): 2521-2532.
  13. Le DT, Uram JN, Wang H, Bartlett BR, Kemberling H, et al. (2015) PD-1 Blockade in Tumors with Mismatch-Repair Deficiency. N Engl J Med 372(26): 2509-2520.
  14. Garon EB, Rizvi NA, Hui R, Leighl N, Balmanoukian AS, et al. (2015) Pembrolizumab for the treatment of non-small-cell lung cancer. N Engl J Med 372(21): 2018-2028.
  15. Nghiem PT, Bhatia S, Lipson EJ, Kudchadkar RR, Miller NJ, et al. (2016) PD-1 Blockade with Pembrolizumab in Advanced Merkel-Cell Carcinoma. N Engl J Med 374(26): 2542-2552.
  16. Zou W, Wolchok JD, Chen L (2016) PD-L1 (B7-H1) and PD-1 pathway blockade for cancer therapy: Mechanisms, response biomarkers, and combinations. Sci Transl Med 8(328): 328rv4.
  17. Wang Q, Wu X (2017) Primary and acquired resistance to PD-1/PD-L1 blockade in cancer treatment. Int Immunopharmacol 46: 210-219.
  18. Grenda A, Krawczyk P (2021) Cancer trigger or remedy: two faces of the human microbiome. Appl Microbiol Biotechnol 105(4): 1395-1405.
  19. Gubin MM, Zhang X, Schuster H, Caron E, Ward JP, et al. (2014) Checkpoint blockade cancer immunotherapy targets tumour-specific mutant antigens. Nature 515(7528): 577-581.
  20. Rizvi NA, Hellmann MD, Snyder A, Kvistborg P, Makarov V, et al. (2015) Cancer immunology. Mutational landscape determines sensitivity to PD-1 blockade in non-small cell lung cancer. Science 348(6230): 124-128.
  21. Schumacher TN, Schreiber RD (2015) Neoantigens in cancer immunotherapy. Science 348(6230): 69-74.
  22. Le DT, Durham JN, Smith KN, Wang H, Bartlett BR, et al. (2017) Mismatch repair deficiency predicts response of solid tumors to PD-1 blockade. Science 357(6349): 409-413.
  23. Martin AM, Nirschl TR, Nirschl CJ, Francica BJ, Kochel CM, et al. (2015) Paucity of PD-L1 expression in prostate cancer: innate and adaptive immune resistance. Prostate Cancer Prostatic Dis 18(4): 325-332.
  24. Marincola FM, Jaffee EM, Hicklin DJ, Ferrone S (2000) Escape of human solid tumors from T-cell recognition: molecular mechanisms and functional significance. Adv Immunol 74: 181-273.
  25. Sucker A, Zhao F, Real B, Heeke C, Bielefeld N, et al. (2014) Genetic evolution of T-cell resistance in the course of melanoma progression. Clin Cancer Res 20(24): 6593-6604.
  26. Zhao L, Yang Y, Ma B, Li W, Li T, et al. (2019) Factors Influencing the Efficacy of Anti-PD-1 Therapy in Chinese Patients with Advanced Melanoma. J Oncol 2019: 6454989.
  27. Zou R, Wang Y, Ye F, Zhang X, Wang M et al. (2021) Mechanisms of primary and acquired resistance to PD-1/PD-L1 blockade and the emerging role of gut microbiome. Clin Transl Oncol.
  28. Wherry EJ, Kurachi M (2015) Molecular and cellular insights into T cell exhaustion. Nat Rev Immunol 15(8): 486-499.
  29. Tumeh PC, Harview CL, Yearley JH, Shintaku IP, Taylor EJ, et al. (2014) PD-1 blockade induces responses by inhibiting adaptive immune resistance. Nature 515(7528): 568-571.
  30. Hanahan D, Coussens LM (2012) Accessories to the crime: functions of cells recruited to the tumor microenvironment. Cancer Cell 21(3): 309-322.
  31. Hugo W, Zaretsky JM, Sun L, Song C, Moreno BH, et al. (2017) Genomic and Transcriptomic Features of Response to Anti-PD-1 Therapy in Metastatic Melanoma. Cell 168(3): 542.
  32. Platanias LC (2005) Mechanisms of type-I- and type-II-interferon-mediated signalling. Nat Rev Immunol 5(5): 375-386.
  33. Gainor JF, Shaw AT, Sequist LV, Fu X, Azzoli CG, et al. (2016) EGFR Mutations and ALK Rearrangements Are Associated with Low Response Rates to PD-1 Pathway Blockade in Non-Small Cell Lung Cancer: A Retrospective Analysis. Clin Cancer Res 22(18): 4585-4593.
  34. Akbay EA, Koyama S, Carretero J, Altabef A, Tchaicha JH, et al. (2013) Activation of the PD-1 pathway contributes to immune escape in EGFR-driven lung tumors. Cancer Discov 3(12): 1355-1363.
  35. Ota K, Azuma K, Kawahara A, Hattori S, Iwama E, et al. (2015) Induction of PD-L1 Expression by the EML4-ALK Oncoprotein and Downstream Signaling Pathways in Non-Small Cell Lung Cancer. Clin Cancer Res 21(17): 4014-4021.
  36. Dzutsev A, Goldszmid RS, Viaud S, Zitvogel L, Trinchieri G (2015) The role of the microbiota in inflammation, carcinogenesis, and cancer therapy. Eur J Immunol 45(1): 17-31.
  37. Kadosh E, Snir AI, Venkatachalam A, May S, Lasry A, et al. (2020) The gut microbiome switches mutant p53 from tumour-suppressive to oncogenic. Nature 586(7827): 133-138.
  38. Shui L, Yang X, Li J, Yi C, Sun Q, et al. (2019) Gut Microbiome as a Potential Factor for Modulating Resistance to Cancer Immunotherapy. Front Immunol 10: 2989.
  39. Allard B, Aspeslagh S, Garaud S, Dupont FA, Solinas C, et al. (2018) Immuno-oncology-101: overview of major concepts and translational perspectives. Semin Cancer Biol 52(Pt 2): 1-11.
  40. Peng Z, Cheng S, Kou Y, Wang Z, Jin R, et al. (2020) The Gut Microbiome Is Associated with Clinical Response to Anti-PD-1/PD-L1 Immunotherapy in Gastrointestinal Cancer. Cancer Immunol Res 8(10): 1251-1261.
  41. Chaput N, Lepage P, Coutzac C, Soularue E, Le Roux K, et al. (2019) Baseline gut microbiota predicts clinical response and colitis in metastatic melanoma patients treated with ipilimumab. Ann Oncol 30(12): 2012.
  42. Routy B, Le Chatelier E, Derosa L, Duong CPM, Alou MT, et al. (2018) Gut microbiome influences efficacy of PD-1-based immunotherapy against epithelial tumors. Science 359(6371): 91-97.
  43. Daillere R, Vetizou M, Waldschmitt N, Yamazaki T, Isnard C, et al. (2016) Enterococcus hirae and Barnesiella intestinihominis Facilitate Cyclophosphamide-Induced Therapeutic Immunomodulatory Effects. Immunity 45(4): 931-943.
  44. Xu X, Zhang X (2015) Effects of cyclophosphamide on immune system and gut microbiota in mice. Microbiol Res 171: 97-106.
  45. Wang L, Tang L, Feng Y, Zhao S, Han M, et al. (2020) A purified membrane protein from Akkermansia muciniphila or the pasteurised bacterium blunts colitis associated tumourigenesis by modulation of CD8(+) T cells in mice. Gut 69(11): 1988-1997.
  46. Zheng Y, Wang T, Tu X, Huang Y, Zhang H, et al. (2019) Gut microbiome affects the response to anti-PD-1 immunotherapy in patients with hepatocellular carcinoma. J Immunother Cancer 7(1): 193.
  47. Zitvogel L, Ma Y, Raoult D, Kroemer G, Gajewski TF (2018) The microbiome in cancer immunotherapy: Diagnostic tools and therapeutic strategies. Science 359(6382): 1366-1370.
  48. Li J, Sung CY, Lee N, Ni Y, Pihlajamaki J, et al. (2016) Probiotics modulated gut microbiota suppresses hepatocellular carcinoma growth in mice. Proc Natl Acad Sci U S A 113(9): E1306-E1315.
  49. Chen D, Jin D, Huang S, Wu J, Xu M, et al. (2020) Clostridium butyricum, a butyrate-producing probiotic, inhibits intestinal tumor development through modulating Wnt signaling and gut microbiota. Cancer Lett 469: 456-467.
  50. Aindelis G, Tiptiri KA, Lampri E, Spyridopoulou K, Lamprianidou E, et al. (2020) Immune Responses Raised in an Experimental Colon Carcinoma Model Following Oral Administration of Lactobacillus casei. Cancers (Basel) 12(2): 368.
  51. Thilakarathna W, Rupasinghe HPV, Ridgway ND (2021) Mechanisms by Which Probiotic Bacteria Attenuate the Risk of Hepatocellular Carcinoma. Int J Mol Sci 22(5): 2606.
  52. Bertrand F, Montfort A, Marcheteau E, Imbert C, Gilhodes J, et al. (2017) TNFalpha blockade overcomes resistance to anti-PD-1 in experimental melanoma. Nat Commun 8(1): 2256.
  53. Scharping NE, Menk AV, Whetstone RD, Zeng X, Delgoffe GM (2017) Efficacy of PD-1 Blockade Is Potentiated by Metformin-Induced Reduction of Tumor Hypoxia. Cancer Immunol Res 5(1): 9-16.
  54. Dudley JC, Lin MT, Le DT, Eshleman JR (2016) Microsatellite Instability as a Biomarker for PD-1 Blockade. Clin Cancer Res 22(4): 813-820.
  55. Munn LL, Jain RK (2019) Vascular regulation of antitumor immunity. Science 365(6453): 544-545.
  56. Du FY, Zhou QF, Sun WJ, Chen GL (2019) Targeting cancer stem cells in drug discovery: Current state and future perspectives. World J Stem Cells 11(7): 398-420.

© 2021 Yongqian Shu. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and build upon your work non-commercially.