RCC-BM poses unique clinical challenges because treatment of BM is complex, and a variety of factors, including anticipated patient survival, competing risks, and long-term toxicities should be considered while selecting the appropriate treatment strategy
[11][17][47]. The brain, being a vital organ, is unable to regenerate upon damage, thus accounting for major limitations for therapy
[11][17][47][48]. For instance, neurosurgery cannot always be performed, and radiotherapy has the risk of irreversibly limiting brain plasticity, which could evolve into a potentially lethal radionecrosis
[48]. Three key indicators favor a good prognosis and thus more aggressive treatment: a KPS score > 70, age < 65 years, and controlled extracranial metastases
[47][49].
The current approach for RCC-BM typically includes surgery (pathologic diagnosis and cerebral decompression) versus standalone radiotherapy and/or systemic therapies, with the overall goal of selecting the optimal treatment for an individual patient to maximize QOL and survival outcomes
[17][47][48][49][50]. Surgery and radiation are the mainstays of therapy and have proven neurological and palliative benefits
[17][47][49][50][51]. Medical therapies for RCC-BM can be divided into two broad categories: symptomatic management and tumor-targeting therapies. Corticosteroids, such as dexamethasone, represent the main symptomatic treatment in addition to pain medications because of their minimal mineralocorticoid effect and control intracerebral edema in BM
[17]; however, the beneficial effects of steroids are not permanent, and a rapid taper is typically recommended to minimize drug-related adverse effects
[17]. In addition, increased understanding of the role of immunosuppression in the pathophysiology of metastatic diseases reveals the potential harm of steroid-associated immunosuppression, thereby encouraging minimal steroid exposure and alternatives to steroid therapy
[17].
4.1. ICIs Based on T Cell Exhaustion in RCC-BM
Although most patients with RCC-BM are excluded from important clinical trials because of poor prognosis and few validated treatment guidelines
[11][48], this trend is diminishing given the increasing importance of clinical significance and a better knowledge of the underlying pathogenesis. The remaining majority of systemic therapies for RCC-BM dramatically changed with the introduction of ICIs and TKIs based on complex microenvironmental niche–tumor interactions, neuroinflammatory cascades, and neovascularization involved in establishing a new BM
[11][17][47][48]. The richness and activation of BM TMEs regarding cellular subtypes, frequencies, and functional states parallels their favorable clinical response to ICIs
[42]. Checkpoint interactions, such as PD-1:PD-L1, CTLA4:B7-1/2, T-cell immunoglobulin and mucin domain-3 (TIM-3):Galectin-9, and lymphocyte activation gene-3 (LAG-3):MHC class Ⅱ, play an important role in immune evasion of cancers
[1]. Drug Administration (FDA) for mRCC include those that block co-inhibitory molecules, such as cytotoxic T-lymphocyte activating protein-4 (CTLA-4), PD-1, and PD-L1, thus facilitating T cell effector function and anti-tumor response
[52][53].
Costimulation with CD28 or 4-1BB can increase anti-tumor activity
[54][55]. CD28 costimulation can increase T cell anabolic metabolism, while the CD28 family members PD-1 and CTLA4 suppress T cell metabolic reprogramming
[54]. CTLA4 inhibits CD28 signaling and PI3K/Akt/mTORC1 signaling, resulting in decreased glycolysis and mitochondrial oxidative capacity
[56]. Blocking the negative regulators of PD-1 and CTLA4 that impair CD28 signaling to inhibit T cell release facilitates anti-tumor activity
[54]. CD8
+ T cells continuously formulate their exhaustion states on account of exposure to suppressive gradients in the TME
[57]. T cell exhaustion is the conversion of the state of CD8
+ T cells from antineoplastic to immune-functionally impaired due to long-term persistence of tumor antigens and/or the suppressive TME
[58][59]. The exhausted CD8
+ T-cell phenotype has been associated with an increased risk of tumor progression
[60][61], increased dysfunctional dendritic cells (DCs)
[62], and elevated numbers of immune cells, namely M2-polarized macrophages, resting mast cells, resting memory CD4
+ T cells, and CD4
+ Foxp3
+ Tregs
[60][63][64][65]. Coinhibitory receptors, such as PD-1 and CTLA-4, are traditionally envisioned as exhaustion markers of T cells
[59][66], which is the theoretical ICB.
Additionally, the prognostic impact of exhausted CD8
+ T cell infiltration in mRCC is only through stratification into specific subgroups
[60][62][64][67][68][69][70]. For example, CXCL13
+ CD8
+ T cells exhibit elevated levels of markers, such as PD-1, Tim-3, T cell immunoreceptor with Ig and ITIM domains (TIGIT) and CTLA-4, higher Ki-67 expression, and lower levels of activated markers, such as tumor necrosis factor α (TNF-α) and interferon γ (IFN-γ)
[67][71]. Furthermore, the abundance of intratumoral CXCL13
+ CD8
+ T cells was positively correlated with immunoevasive TME accompanied by increased T helper 2 cells, TAMs, CD4
+ Foxp3
+ Tregs, and decreased NK cells
[67]. The HIF-1-TGF-β pathway might serve as a crucial molecule in connecting CXCL13
+ CD8
+ T cells and TME,
[67][72][73][74]. Neoantigen reactivity is coupled to a CXCL13-secreting ‘‘exhausted’’ phenotype, possibly induced by chronic TCR signaling
[75]. The selective expression of CCR5 and CXCL13 in neoantigen-specific T cells further suggests that a key feature of ICI-responsiveness is the ability to sustain ongoing priming and recruitment of tumor reactive T cells supported by CXCR5
+ lymphocytes
[76][77]. Interestingly, patients with higher numbers of CD39
+ CD8
+ T cells showed improved responses to sunitinib, a multi-TKI, suggesting that evaluation of the exhausted phenotype for CD8
+ T cells may help in clinical decision making or therapy selection
[61].
Many receptors in the immunoglobulin superfamily (such as CD28, and inducible T cell co-stimulator) and TNF receptor superfamily (TNFSF) exert costimulatory actions
[78]. TNFSRF9 is thought to be an antigen stimulation-inducible co-stimulatory receptor, which is transiently expressed on activated CD8
+ T, activated CD4
+ T, and NK cells
[64][79][80]. Co-stimulatory signaling mediated by TNFRSF9 promotes T cell proliferation, secretion of cytokines, resistance to activation-induced cell death, and development of memory T cells
[80]. TNFRSF9
+ CD8
+ T cells possess both exhaustion (PD-1, TIM-3, CTLA-4, and TIGIT) and effector phenotype (IFN-γ, granzyme B, and Ki-67)
[79]. This dual phenotype of TNFRSF9
+ CD8
+ T cells indicates that these cells may not be terminally exhausted; however, they could respond to ICB
[79]. The functional status of TNFRSF9
+ CD8
+ T cells might partly result from the complicated interactions among immune cells (helper T cells, CD8
+ T cells, and myeloid cells) within the tumor, and high enrichment of TNFRSF9
+ CD8
+ T cells could be a predictor of immunotherapy and a novel therapeutic target in mRCC
[79].
4.2. ICIs Based on Targeting Immunometabolomics in RCC-BM
RCC is essentially a metabolic disease characterized by a reprogramming of energetic metabolism, and many genes that are mutated in RCC encode proteins that have roles in cellular processes regulating oxygen and glucose consumption
[54]. In particular the metabolic flux through glycolysis is partitioned
[81][82][83][84], and mitochondrial bioenergetics and oxidative phosphorylation are impaired, as well as lipid metabolism
[82][85][86]. In addition, RCC is one of the most immune-infiltrated tumors
[87][88]. Emerging evidence suggests that the activation of specific metabolic pathway have a role in regulating angiogenesis and inflammatory signatures
[89][90]. Features of the TME heavily affect disease biology and may affect responses to systemic therapy
[91]. VHL mutations that occur in mRCC increased transcriptional activity of its target genes, such as
VEGF, glucose transporter 1, and erythropoietin, independent of oxygen levels, promoting angiogenesis, and immunosuppression
[54]. The complexity of cellular interactions and depletion of available nutrients may create an environment of nutrient competition for T cells, and buildup of waste products that may impair T cells
[92]. RCC-BM demonstrated metabolic changes leading to alterations in pathways associated with energy metabolism and oxidative stress, as well as the accumulation of immunosuppressive metabolites, such as tryptophan (TRP)
[54][92]. Enhanced activity across an array of interconnected oncogenic signaling networks centered on the PI3K-AKT pathway represents a generalizable feature across different BM histologies
[92].
The analysis of metabolic pathways intrinsic to immune cell types, also known as immunometabolism, could identify markers of immune function based on the distinct metabolic requirements of these cells at each stage of differentiation
[93]. At the single-cell level, costimulation shifted the percentage of cells from a baseline resting state into two primary branches: one that was enriched in IL-2 signaling and glycolysis and another that exhibited pathways of glycolysis, oxidative phosphorylation, and Myc signaling
[54]. This bioenergetic switch is consistent with the known Myc regulation of metabolic reprogramming during T cell activation
[54]. Activation, together with signaling through the costimulatory molecule CD28, augments signaling through the PI3K/Akt/mTORC1 pathway to increase glucose and mitochondrial metabolism, and enable robust proliferation and effector function
[94][95].
Metabolic reprogramming dictates the fate and function of stimulated T cells and microenvironment of tumors coupled with chronic exposure to neoantigens can impair the metabolism of TILs
[54][96][97]. Stimulated T cells are highly dependent on metabolic reprogramming from catabolic oxidative metabolism to anabolic metabolism with elevated glucose consumption and aerobic glycolysis to develop effector functions
[98][99][100]. T cell activation leads to increased Myc and PI3K/Akt/mTORC1 signaling activity to promote glucose uptake and mitochondrial metabolism for growth and energetics, and to regulate signaling and gene expression pathways
[95][101][102]. CD8
+ T cells in RCC can be subject to metabolic barriers that lead to adaptations, such as reduced ability to absorb glucose for downstream glycolysis, fragmented and functionally altered mitochondria with low respiratory capacity, and elevated production of reactive oxygen species (ROS)
[97][103]. These changes are critical for effector T cell function, as CD8
+ PD-1
+ cells subject to inhibition of glucose metabolism fail to develop into effector subsets and have a reduced capacity to favor suppressive Treg fates
[54][103]. RCC CD8
+ TILs have altered metabolic and functional parameters, suggesting reduced metabolism and failure of antigen receptor stimulation to activate a predominant effector memory phenotype
[54].
In addition, PD-1 signaling suppresses T cell metabolic reprogramming by inhibiting glycolysis and promoting lipolysis and fatty acid oxidation (FAO)
[54][56]. While CD8
+ RCC TIL gene expression exhibits classical markers of chronic stimulation and enrichment of metabolic pathways, including FAO, glycolysis, and cholesterol homeostasis, a large portion of cells could be stimulated to reprogram metabolism and induce effector functions
[54]. The link between very long chain fatty acid-containing lipids and response to ICI in RCC can be explained by enhanced peroxisome signaling in activated T cells, which leads to a metabolic switch to fatty acid catabolism
[104]. Lastly, increased conversion of TRP to kynurenine by IDO leads to inhibition of T cell function and is involved in the regulation of the immunosuppressive TME of mRCC
[54][105].