DNT cells account for only 3–5% of peripheral blood T-lymphocytes, and they play crucial roles in both innate and adaptive immune responses [20]. In addition, DNT cells are closely related to autoimmune diseases, inflammatory infections, and tumor progression [21]. Some studies reported an increased percentage of DNT cells in hematological and solid tumors. For instance, the proportion of TCR-αβ+ DNT cells is significantly higher in the peripheral blood of patients with acquired aplastic anemia than in healthy controls, and these patients are more responsive to immunosuppressive therapy [22]. In our study, we similarly found that the percentage of peripheral DNT cells was higher in patients with BC than in healthy subjects. There is considerable ambiguity regarding the function of DNT cells in different types of tumors. For example, although DNT cells have been revealed to exert immunosuppressive functions in mouse melanoma and glioma models, they have the ability to inhibit the growth of malignant cells in most tumors [23]. In vitro-expanded DNT cells induced cytotoxic effects in co-cultured triple-negative BC cells through the classic cell-killing mechanisms, including direct interaction via NKG2D and DNAM-1 and secretion of perforin and granzyme B. Therefore, we further explored the function and potential mechanism of DNT cells in BC by full-length transcriptome sequencing and bioinformatic analysis.
We identified 289 DEGs, including 137 upregulated and 152 downregulated genes, in DNT cells from patients with BC. Per the results of GO enrichment analysis, the DEGs are likely involved in immunoglobulin mediated humoral immune responses, the classical pathway of complement activation, and the B cell receptor signaling pathway. KEGG analysis also revealed significant enrichment of the B cell receptor signaling pathway and the complement and coagulation cascade cell signaling pathways among these DEGs. Taken together, these results suggest a close association between DNT cells and B cells. DNT cells are key coordinators of the immune response, and they might play a role in tumor immunity by regulating B cells.
The results of GO and KEGG enrichment analyses also demonstrated the involvement of DEGs in the classical complement activation pathway. In addition, the complement components C1QB and C1QC were identified as hub genes, and they were highly expressed in the BC group. The complement system is an important branch of innate immunity [24], and complement activation has been revealed to play a role in tumor progression [25]. C1Q promotes T cell activation and IFN-γ production and facilitates the subsequent immune response by activating the complement classical pathway [26, 27]. IFN-γ stimulates cellular immune responses and mediates the anti-tumor effects of DNT cells [21, 28]. Therefore, we hypothesized that C1Q activates DNT cells in the breast tumor microenvironment and triggers an anti-tumor immune response.
The PPI network comprised 2 modules and 10 hub genes, of which TMEM176A and TMEM176B displayed the most significant differences in expression between the BC and control groups. TMEM176B was significantly downregulated in patients with BC compared to the healthy controls. It encodes a cation channel protein of the MS4A transmembrane 4A family, and it is mainly expressed on lymphocytes and hematopoietic cells [29]. Segovia et al. found that TMEM176B controls the pH in phagosomes by modulating cation currents, which in turn affects antigen cross-presentation by dendritic cells [30]. Ion channels also play an important role in enhancing anti-tumor immunity as regulatory checkpoints and therapeutic targets [31]. Therefore, TMEM176B might influence the differentiation and immunomodulatory function of DNT cells, and the higher percentage of DNT cells in patients with BC relative to healthy controls could be related to difference in TMEM176B expression between the two groups. In addition, TMEM176B directly promotes the growth of triple-negative BC cells by activating the AKT/mTOR signaling pathway, and TMEM176B blockade using monoclonal antibodies significantly inhibited cancer cell proliferation. Taken together, DNT cells might exert an anti-tumor effect in BC by inhibiting TMEM176B.
EGR1, a transcriptional regulator containing a zinc-finger DNA-binding domain, is poorly expressed in resting T cells, and its expression rapidly increases upon the induction of TCR signaling [32]. The TCR signaling cascade regulates T cell proliferation, survival, and differentiation. EGR1 binds to the T-bet promoter homeostatic element and induces T-bet transcription, thereby activating and synergizing the TCR signaling pathway [33]. Furthermore, EGR1 can activate TNF-α and FasL transcription in response to TCR signaling [34, 35]. Several studies revealed that DNT cells exert anti-tumor effects by promoting the secretion of TNF-α, IFN-γ, and FasL [21]. In this study, EGR1 was highly expressed in the BC group compared to the healthy controls. This led us to hypothesize that EGR1 promotes the growth and differentiation of DNT cells via the TCR signaling pathway, which in turn elicits an anti-tumor immune response.
IFIT1, IFIT3, IFI27, IFI44L, and RSAD2 are interferon-stimulated genes (ISGs) that are highly expressed in a subset of patients with BC [36], and they belonged to module 1 in the PPI network. ISG-encoded proteins are induced by type I IFN signaling, and they regulate innate and adaptive immune responses involved in the development of multiple autoimmune diseases and tumors [37]. Type I promotes the survival of CD8+ T cells and enhances their anti-tumor capacity [38]. Thus, DNT cells might regulate the immune system by activating the IFN signaling pathway, thereby enhancing the anti-tumor immune response.