TLSs are lymphoid neogenesis that occur in nonlymphoid tissues, such as TME, under chronic inflammation and immune stimulation. [17] Currently, TLS detection is mainly performed by IHC analysis of pathological sections, which is valuable for definitively determining the presence of TLSs. However, there are several limitations of IHC-based TLS section analysis, such as the consumption of excessive clinical sample resources, inconvenience in quantitative analysis, complexity in combination analysis with TME, and difficulty in performing underlying mechanism elucidation. [9, 10] Therefore, there is a robust demand for the establishment of TLS gene signatures using transcriptomic sequencing data, which will be more suitable for TLS-related clinical and basic researches.
Previously, scholars have attempted to identify TLS neogenesis and TLS presence-related biomarkers. First, CCL19, CCL21, CXCL12, and CXCL13 were defined as the main chemokine genes associated with TLSs. [18] TLSs are composed of mature DCs, the T cell zone and the B lineage zone, indicating the occurrence of lymphoid neogenesis as the secondary lymphoid organs adapted to inflammatory signals. [10] Based on the cellular composition in TLSs, a 12-chemokine signature (CCL2, CCL3, CCL4, CCL5, CCL8, CCL18, CCL19, CCL21, CXCL9, CXCL10, CXCL11, CXCL13) and 9 gene signature (CCL19, CCL21, CXCL13, CCR7, SELL, LAMP3, CXCR4, CD86, BCL6) were proposed with the conclusion that this signature showed an extremely heterogeneous distribution among different cancer types.[13, 14] More specifically, some tumor types, such as lung adenocarcinoma, squamous cell carcinoma, glioblastoma and uveal melanoma, presented with high level of TLSs. However, other malignant tumors, including adrenocortical carcinoma, paraganglioma and pancreatic cancers, showed a relatively low proportion of TLSs. [10] These findings suggested that the TLS signature should be applied to the specific TME due to the high heterogeneity. We integrated chemokines reported by other cancer types and performed screening by clustering classification and eliminating the genes with low correlation, and established a 13-gene signature for HNSCC. Through prognostic analysis and ICB therapy verification, we proved the rationality of the current TLS 13-gene signature in HNSCC.
The presence of TLSs was associated with a lower risk of early recurrence of hepatocellular carcinoma [9, 19], TNM stage refinement in non-small cell lung cancer,[20] immunological strategies for triple‑negative breast cancers [21]and upper tract urothelial carcinoma, [22] and was also found to be an innovative prognostic biomarker for HNSCC. [5, 12] What’s more, TLSs mainly participate in adaptive immune responses and could be deemed as prognostic and predictive factors. To expound the correlation between TLSs and immunotherapeutic responses, clinical trial-related ICB-treated cohorts, including primary and metastatic melanoma (NCT02519322, NCT02437279), renal cell carcinoma (NCT02210117) and soft tissue sarcoma (SARCO28) cohorts, have been investigated and revealed that B lineage cell-enriched TLSs are ideal biomarker for ICB therapy efficacy prediction and are valuable for precise clinical decision making. [6–8] In a cohort of breast cancer with 1058 patients, the high density of both T and B lymphocytes within TLSs in pretreatment biopsy samples was positively correlated with pCR following neoadjuvant immune therapy. [23] This predictive role of TLSs on the immune response may be ascribed to the TME modulation induced by TLSs neogenesis. However, the influence and underlying mechanism of TLS neogenesis on the TME modulation, especially the immune cell composition and immune responses, have been preliminarily deciphered. The presence of TLSs may signify that tumor antigens are recognized by the antigen presenting cells and further T lymphocytes. TLS-hi tumors are characterized by increased infiltration of activated T cells, memory-type CD8 + T cells and TH17 cells with enhanced chemotaxis and cytotoxicity, and decreased proportion of immunosuppressive cells. [24] TLSs in tumors also hold the capacity in the education of intratumoural T cells into effector and memory statues, which verifies that TLSs support the activation of CD8 T cells and further attacking against tumor cells, resulting in the best survival outcome in tumors with the presence of both TLSs and CD8 T cells. Conversely, CD8 T cells in tumors without TLSs infiltration showed high expression level of TIM-3 and PD-1, indicating that those CD8 T cells were more prone to develop into exhausted statue and dysfunctional molecular phenotype, which failed to response to ICB. [6]
Few studies have focused on the association analysis of TLSs with T cell exhaustion in the TME. In line with the findings in triple-negative and HER2 + subtypes of breast cancer, a higher proportion of exhaustive T cells was also found in HNSCC samples. [25] Reviewing the HNSCC data, we found that a high density of T cells or CD8 + T cells may contribute to the higher proportion of exhausted T cells. The exhausted phenotype of T cells could be deemed as a special stage or status of T cells; thus, it is not difficult to understand the higher infiltration rate of exhausted T cells in the immune inflamed microenvironment. In addition, the remaining or surviving malignant tumors in this high immune-infiltrated TME may be more able to induce T cells to transform into exhausted statue.
In addition, TLSs enrichment was positively associated with an augment of intratumor CD4 T cells clonality, characterized by higher infiltration of naïve, central-memory, and activated CD4 T cells and lower distribution of regulatory T cells, which indicated that TLSs neogenesis may be able to narrow the deleterious impact of high Treg density on immune response.[26] In HNSCC, we found a lower abundance of memory-type resting CD4 + T cells, higher infiltration of memory-type activated CD4 + T cells, and no distinct difference in Tregs and T helper cells. The CD20 + B lineage is a representative cell within TLSs. Memory-type but not naïve and plasma-type B cells significantly accumulated in TLS-hi tumors. TLSs are a privileged area responsible for the generation of effector memory cytotoxic cells and memory B cells, thus sustaining a long-term immune response against tumor antigens and promoting further antibody production.[27] In single cell analysis, uniform elevation of MHC-I and MHC-II in B cells indicated that B lymphocytes within TLSs are capable of antigen presentation. B cells enriched tumors contained more CD 4 and CD8 T cells with naïve and memory features, and CD20 + B cells colocalized with CD4/8 T cells in TLSs of tumors of ICB responders, such synergistic effect of B cells and T cells promote the ICB therapeutic efficacy. [6, 7] Besides T and B cells, TLS-associated mature DCs yield a specific immune compound featured by cytotoxic orientation, thus shaping the immunophenotype of TME and promoting a protective immune response regulated by T cells against cancer. [24] In HNSCC, a higher proportion of activated DCs was also detected in TLS-hi tumors, indicating that TLSs may favor DC priming and activation, which further elicits the antitumoral response of CD8 + T cells. [28, 29] What’s more, TLSs also favor surrounding high endothelial venules (HEVs) in lymphocyte recruitment, and the combination of antiangiogenic and anti-PDL1 therapies increased HEV formation, thus further improving antitumor immune responses. [10]
Vaccine trials also illustrated that TLSs may be a major component in the induction of antitumor immunity. Therapeutic vaccination of cancer patients induced the TLSs neogenesis and proliferating lymphocytes in the stroma adjacent to the tumor region, which were not found in non-vaccinated patients. Expression gene and pathway analysis of micro-dissected TLSs showed upregulation of immune cell activation and trafficking, Th17 cell stimulation, and suppressed function of Treg cells. [10] These discoveries demonstrated the potential role of TLSs in converting non-immunogenic TME into immune inflamed tumor ecosystem.
The classical immune classification of the TME is “immune infiltrated”, “immune excluded” and “immune desert”, which was proposed on the basis of the infiltration levels and distribution of CD8 + T cells and other immunosuppressive cells. [8] With the development of bioinformatic techniques, an increasing number of classifications have been proposed, including molecular subtypes based on the gene signature, the combination of immune features with epigenetic information, and the immune cell infiltration landscape. [30, 31] In soft tissue sarcoma, TME compositions were integrated with TLSs to form the “SICs” classification: A, immune desert; C, vascularized; E, immune and TLS high; B and D, immune-low and high profiles. However, only SIC E was associated with TLS status.[8] Due to the high heterogeneity of tumors, this classification cannot be used for other types of cancers. Here, we integrated immunological parameters, stromal cells and score, TLSs, TMB and malignant cells to establish a novel TME classification for HNSCC, through which we proved that infiltrated stromal cells may counteract the high proportions of immune-promoted cells. It is not difficult to understand why the worst survival rate was observed in HNSCC-TC 4 patients as this subtype is characterized by an immune desert and a high density of stromal cells. The co-existence of immune promoted and immunosuppressive cells, high infiltration of T cells and exhausted T cells, Tregs cells is also of importance. We also found an interesting phenomenon that no significant difference in TLSs level exists between the CR and SD groups. This diversity in therapeutic efficacy may be associated with different prognoses between HNSCC-TCs 3 and 5, which were characterized by identically high levels of TLSs but different stromal scores, indicating that PD tumors may have a higher density of stromal cells, which may facilitate immune evasion.
In conclusion, we established a gene signature containing 13 chemokines to identify the TLS level in HNSCC samples, and then explored the correlation of TLSs with immune check point and immune landscape. Based on these, we further confirmed the clinical significance of TLSs, especially for T cell and B cell. Through gene profiling analysis, we uncovered the underlying mechanism of TLSs in remodeling the HNSCC TME, and preliminarily discussed the potential role of innate immune cells and CAFs in the neogenesis of TLSs, which may provide a pivotal guide for TME intervention. What’s more, combining the immune cells and score, stromal cells and score, TLSs and malignant cells, we established a novel classification of HNSCC. Finally, the predictive role of TLSs in immunotherapy has been verified in ICB-treated cohorts. Further consideration should be paid attention to the verification of TLSs signature in prospective cohort studies with regard to ICB treatment.