The clinical implementation of immune checkpoint inhibitors, particularly monoclonal antibodies targeting the programmed cell death protein 1 (PD-1) pathway, has transformed the oncological landscape. Despite significant durable responses in various malignancies, a substantial proportion of patients exhibit primary resistance, while others develop acquired resistance after an initial period of benefit. The mechanisms underlying this immune escape are multifactorial, involving tumor-intrinsic genetic alterations, the presence of immunosuppressive cells within the tumor microenvironment (TME), and the activation of alternative inhibitory pathways. Understanding these complex interactions is vital for the development of next-generation combination therapies. In this pursuit, we facilitate advanced research by providing flexible and reliable analysis services using protein array technology, designed to meet diverse research needs in cytokine profiling and biomarker discovery. This review synthesizes current knowledge on the molecular and cellular drivers of resistance to anti-PD-1 therapy, offering insights into potential strategies to overcome these barriers.
Introduction
The PD-1/PD-L1 axis serves as a critical immune checkpoint that maintains peripheral tolerance and prevents autoimmunity under physiological conditions. However, many tumors hijack this pathway by upregulating programmed death-ligand 1 (PD-L1), thereby inducing T-cell exhaustion and blunting the anti-tumor immune response. While anti-PD-1 therapies aim to reinvigorate these exhausted T-cells, the efficacy of this approach is often hampered by resistance. Resistance is generally categorized into primary (no initial response), adaptive (immune response is recognized but the tumor adapts), and acquired (relapse after an initial response). The escape mechanisms are broadly divided into tumor-intrinsic factors, such as the loss of antigen presentation, and tumor-extrinsic factors, primarily involving the metabolic and cellular constraints of the TME.
Tumor-Intrinsic Mechanisms of Resistance
One of the most fundamental requirements for an effective anti-PD-1 response is the presence of tumor antigens that can be recognized by T-cells. Tumor-intrinsic resistance often begins with a lack of immunogenicity. Tumors with a low tumor mutational burden (TMB) generate fewer neoantigens, making them "invisible" to the immune system. Beyond mutational load, specific genetic alterations can actively exclude T-cells or impair their function. For instance, the activation of the WNT/β-catenin signaling pathway has been closely linked to the exclusion of T-cells from the tumor site. Elevated β-catenin levels lead to decreased expression of CCL4, a chemokine necessary for the recruitment of CD103+ dendritic cells, which are essential for priming CD8+ T-cells.
Furthermore, the loss of target antigens or defects in the antigen-processing and presentation machinery (APM) represents a major escape route. The loss of Beta-2 microglobulin (B2M) is a well-documented mechanism of acquired resistance. B2M is required for the stable expression of Major Histocompatibility Complex (MHC) class I molecules on the cell surface; its absence prevents T-cells from recognizing tumor cells, regardless of PD-1 blockade. Similarly, mutations in JAK1 and JAK2 kinases, which are part of the interferon-gamma (IFN-γ) signaling pathway, can render tumor cells insensitive to the anti-proliferative effects of IFN-γ and prevent the reactive upregulation of MHC molecules, thereby facilitating immune evasion.
Figure 1. Mechanism of melanoma resistance to PD-1 inhibitors. (Source: Du F, et al. 2023)
The Role of the Tumor Microenvironment (TME)
Even when T-cells are successfully primed and recruited, the extrinsic environment of the tumor can pose significant hurdles. The TME is often a hostile, acidic, and hypoxic space that suppresses effector T-cell function. Metabolic competition for essential nutrients like glucose and amino acids (e.g., tryptophan) is a key feature of immune escape. The expression of Indoleamine 2,3-dioxygenase (IDO) by tumor or myeloid cells depletes tryptophan and produces kynurenine, which directly inhibits T-cell proliferation and promotes the differentiation of immunosuppressive regulatory T-cells (Tregs).
The cellular composition of the TME is equally critical. Infiltrating immunosuppressive populations, such as Myeloid-Derived Suppressor Cells (MDSCs) and Tumor-Associated Macrophages (TAMs), produce a variety of inhibitory cytokines including TGF-β and IL-10. TGF-β, in particular, promotes a "desmoplastic" phenotype, characterized by dense extracellular matrix (ECM) deposition that physically blocks T-cells from infiltrating the tumor parenchyma. This "immune-excluded" phenotype is a common barrier to anti-PD-1 efficacy. Furthermore, TAMs can express other inhibitory ligands and contribute to the metabolic exhaustion of effector cells, creating a robust shield against immunotherapy.
Figure 2. Immunosuppressive functions of MDSCs in the tumor microenvironment. (Source: Yang Z, et al. 2020)
Adaptive Resistance and Alternative Checkpoints
In many cases, the immune system initially responds to PD-1 blockade, but the tumor undergoes adaptive changes to survive. A common mechanism of adaptive resistance is the compensatory upregulation of alternative immune checkpoints. When the PD-1 pathway is blocked, the immune system may increase the expression of other inhibitory receptors such as TIM-3, LAG-3, TIGIT, or VISTA. These receptors provide redundant signals that maintain the state of T-cell exhaustion. Chronic exposure to tumor antigens under PD-1 blockade can drive T-cells into a terminally exhausted state that is epigenetically fixed and resistant to further reinvigoration.
Moreover, epigenetic modifications within the tumor cells can silence the expression of genes involved in the immune response. DNA methylation and histone acetylation changes can lead to the downregulation of MHC molecules or the silencing of Th1-type chemokines (e.g., CXCL9, CXCL10), thereby reducing the recruitment of effector cells. The dynamic nature of these epigenetic shifts allows tumors to rapidly adapt to the selective pressure exerted by the immune system during therapy.
Figure 3. The immunosuppression mechanisms underlying LAG-3 action in the TME. (Source: Cai L, et al. 2023)
Future Perspectives and Combinatorial Approaches
Overcoming resistance to anti-PD-1 therapy requires a multi-pronged approach tailored to the specific escape mechanism present in each patient. For "cold" tumors lacking T-cell infiltration, strategies to enhance immunogenicity, such as the use of oncolytic viruses, vaccines, or chemotherapy to induce immunogenic cell death, are being explored. To address the metabolic constraints of the TME, inhibitors targeting IDO, adenosine receptors, or TGF-β signaling are currently in various stages of clinical evaluation.
The most promising strategy involves the combination of anti-PD-1 with agents targeting alternative checkpoints like LAG-3 or TIGIT. By blocking multiple inhibitory signals simultaneously, it may be possible to prevent or reverse T-cell exhaustion more effectively. Additionally, the integration of precision medicine, utilizing biomarkers like B2M status or IFN-γ gene expression signatures, will be essential for identifying patients likely to benefit from specific combinations. As our understanding of the systemic and local factors driving immune escape continues to evolve, the goal of achieving durable responses in a broader patient population becomes increasingly attainable.
Frequently Asked Questions (FAQs)
Primary resistance refers to a situation where a patient's tumor never responds to the therapy from the outset, often due to a lack of pre-existing immune infiltration or low immunogenicity. Acquired resistance occurs in patients who initially showed a clinical response (partial or complete) but later experienced disease progression, often due to the tumor developing new escape mutations, such as B2M loss.
"Cold" tumors are characterized by a lack of T-cell infiltration (immune-desert tumors). Since anti-PD-1 therapy works by reinvigorating T-cells already present in the tumor vicinity, it cannot be effective if there are no T-cells to act upon. This is often caused by low mutational burden or active signaling pathways that exclude immune cells.
B2M (Beta-2 microglobulin) is an essential structural component of MHC class I molecules. When a tumor cell loses B2M, it can no longer present antigens on its surface. Without antigen presentation, CD8+ cytotoxic T-cells cannot "see" the tumor cell, rendering the immune response—and the therapy—ineffective.
Yes, combination therapies targeting multiple checkpoints are a major area of current research. By blocking both PD-1 and receptors like LAG-3 or TIM-3, researchers hope to overcome the redundant inhibitory signals that tumors use to maintain T-cell exhaustion, potentially reversing resistance.
References
- Du F, et al. The role of mitochondria in the resistance of melanoma to PD-1 inhibitors. J Transl Med. 2023 May 23;21(1):345.
- Yang Z, et al. Myeloid-derived suppressor cells-new and exciting players in lung cancer. J Hematol Oncol. 2020 Jan 31;13(1):10.
- Cai L, et al. Targeting LAG-3, TIM-3, and TIGIT for cancer immunotherapy. J Hematol Oncol. 2023 Sep 5;16(1):101.
- Ribas A, et al. Cancer immunotherapy using checkpoint blockade. Science. 2018 Mar 23;359(6382):1350-1355.
- Sharma P, et al. Primary, Adaptive, and Acquired Resistance to Cancer Immunotherapy. Cell. 2017 Feb 9;168(4):707-723.