For the first time, our study identified an increased proportion of CD8+ TRM-like cells in the BM of patients with ND-AML. A high proportion of CD8+ TRM-like subset is associated with poor OS. TRM cells are a nonrecirculating memory T-cell lineage that persists in tissues [26, 42]. They were originally discovered for their ability to fight viral reinfection and have recently gained attention for their role in the fight against cancer [43, 44, 45, 46, 47]. The main characteristics of TRM cells are their expression of tissue-resident markers CD103 and CD69 [42, 48] and their lack of egress-associated molecules such as CD62L, CCR7, and S1PR1 [26]. The expression of CXCR6, fatty acid-binding protein, PD-1, CTLA-4, and CD39 can also be used to identify TRM subpopulations in normal tissues or tumors [4, 49, 50, 51]. From a transcriptional perspective, TRM cells in cancer are also defined by a high expression of ZFP683 (HOBIT), RUNX3, FOS, BHLHE40, and NR4A-family members and a low expression of KLF2 and Eomes [26, 46, 52, 53, 54]. TRM cells with these characteristics can infiltrate a variety of solid tumors [32, 51]. An increased proportion of CD8+ TRM-like cells in several tumor tissues has been associated with improved survival. For example, in lung and skin cancers, CD8+ TRMs have a phenotype with high PD-1 expression; produce Th1 cytokines such as IFN-γ, TNF-α, and IL-17; and are associated with significant improvement in patient status, suggesting that TRMs have a special function in controlling tumor progression [35, 55]. Although TRM cells express immune checkpoint receptors in melanoma, such as PD-1, they can also secrete IFN-γ and TNF-α, providing evidence for their antitumor immune response [56]. Furthermore, TRM cells from a variety of primary ovarian tumors, including endometrioid, mucinous, clear cell, and high-grade serous carcinoma, express substantial amounts of T-cell restricted intracellular antigen-1 with cytotoxic potential, although they also highly express PD-1 [57]. These positive correlation examples seem to have a similar mechanism, that is, even though TRM cells highly express several immune checkpoints and display exhaustion phenotypes, they are endowed with relatively high cytotoxic capacity and could limit the progression of tumors. In the present study, the BM TRM-like subset also exhibited the characteristics mentioned above, such as a high percentage of PD-1, TIGIT, and TOX and a median cytotoxicity score. However, the opposite survival correlation was observed in AML. Similar results were found in the study of Zhou et al. [58] in prostate cancer, CD103 expression conferred negative prognostic impact and immunosuppressive function to tumour-infiltrating CD8 T cells, while the CD103 CD8 T cells exhibited a powerful anti-tumour immunity. We hypothesized that this phenomenon may be related to the special BMM of hematological tumors. T Baldridge et al. reported that the increased release of IFN-γ can travel through the bloodstream to activate HSCs in the BM, leading to expansion and mobilization of the immune progenitor pool [59]. Memory T cells can support HSC self-renewal and contribute to their maintenance and/or recovery by producing soluble factor(s) [60]. Therefore, infections could elicit an increase in CD8+ TRM cell population in the BM to support subsequent tissue repair, which in turn supports hematopoiesis and stimulates myelopoiesis [61]. In leukemia, the accumulated TRM cells expressing immune inhibitory molecules may not only kill the leukemic cells but even support the proliferation of leukemia stem cells or promote myeloid leukemia cell proliferation, especially in the M4 and M5 subtypes. However, this hypothesis still needs further validation by experiment. In addition, other mechanisms cannot be excluded. For example, the accumulation of atypical tissue resident CD69+ terminal effector cells (CD8+CD57+) in myeloma-infiltrated BM could prevent the differentiation and expansion of clonal myeloma-specific CD8+ terminal effector cells and ultimately contribute to myeloma immune escape [62]. In the recent work of Ryo Koyamaet al. [63], they found that CD69 deficiency resulted in a decreased expression of the transcription factor TOX in tumor-specific CD8+ T cells in tumor-draining lymph nodes, promoting the differentiation of stem-like CD8+ T cells into functional terminally differentiated CD8+ T cells with enhanced antitumor function and synergizing with anti-PD-1 treatment to render immune refractory B16 melanoma susceptible to immunotherapy. The above results indicated that CD69 acts as an important negative regulator for the differentiation of tumor-specific CD8+ T cells. Therefore, targeting CD69 and PD-1 may be a promising immunotherapy for AML. A recent work also reported that tumor-specific or bystander TRM-like cells present before tumor onset shape the evolution of tumor immunogenicity and boost immune cell recruitment, causing tumor immune evasion through the loss of MHC class I protein expression and resistance to ICIs [64]. BM is a TRM-like cell abounded field; whether the poor prognosis of patients with a high CD8+ TRM-like proportion may also be attributed to the loss of MHC class I caused by high immune pressure is worthy of further investigation.
This study also found that the CD8+ TRM-like cells in the BM highly express several costimulatory molecules CD27 and CD28 and lowly express Ki-67, a proliferation marker, and CD57, a marker of replicative senescence and terminal differentiation. Single-cell proteogenomic analysis also revealed that the CD8+ TRM-like subset in the BM highly expresses several other costimulatory molecules such CD278 and CD137. These results indicated that BM D8+ TRM-like cells may remain in a quiescent state without seriously replicating but maintains a potential to differentiate and give rise to a huge population of T cell progeny. Brahma V et al. [65] also reported that human lung and spleen CD8+CD69+ memory T cells undergo reduced proliferation turnover, which is indicated by a low expression of Ki-67 and CD57. Thomas Gebhardt [66] proposed that true TRM cells still retain a certain degree of stemness, allowing them to maintain self-proliferation and differentiate into circulating T cells upon encountering antigens. For example, one longitudinal study found that tumor-reactive bona fide TRM cells could be maintained in the skin of melanoma survivors free of overt disease for up to 9 years [67]. Although TRM cells have superior proliferation potential, ICR expression is another feature that may limit their activation. Conversely, exhausted T cells could acquire the tissue-residency features of TRM cells to adapt to the tissue microenvironment, especially in the TME. Thus, using ICIs to activate the antitumor potential of tumor-infiltrating TRM-like cells is a reasonable method for tumor treatment. In fact, several studies reported that TRM or TRM-like cells could directly respond to immune checkpoint blockade by producing effector-like cytolytic exhausted T cells, exhausted T cells in transient or intermediate differentiation states, and progeny downstream cells of terminally exhausted T cells, directly attacking cancer cells to further promote tumor control [55, 68, 69, 70]. This effect may be attributed to the high expression of costimulatory molecules of TRMs. As reported by Kim et al. [71], the expression of costimulatory receptors in CD8+ T cells is an important factor for responsiveness to PD-1 blocking. Qureshi et al. [72] also stated that CTLA-4 inhibits T cell activation by removing CD80/CD86 from APC, followed by transendocytosis and degradation. Meanwhile, PD-1 mediates its effects by inhibiting signaling via the interaction of SHP-2 with an ITIM or an ITSM in its cytoplasmic domain [73, 74]. If blocking CTLA-4 or PD-1 can increase CD8 TEM cell proliferation, then this proliferation can be blocked with an anti-CD28 domain antibody. Thus, the TRM-like cells infiltrating tumor tissues may be the real responsible target cell in ICI treatment. For AML and other cancers without significant response to ICI treatment, the successful application of ICIs relies on the discovery of a second immune suppressive brake that functions independently with ICRs.