Antitumor effects of TIM-1 blockade in B cells

A research by Bod et al.¹ has been published recently in Nature entitled “B-cell-specific checkpoint molecules that regulate anti-tumor immunity‘.” It revealed that T-cell immunoglobulin and mucin domain protein 1 (TIM-1) blockade can strikingly reduce the melanoma size in mouse models.

In the field of antitumor immunotherapy, researchers have been extensively studying the role of T cells in fighting against tumors. However, the function of B cells in antitumor immunity has remained unclear or controversial. To address this gap, this collaboration study among Harvard, MIT, and many others has shed light on a crucial molecule called TIM-1, which for the first time, has been claimed to act as a specific checkpoint on B cells.¹

TIM-1 is also known as hepatitis A virus cellular receptor 1 (HAVCR1) and kidney injury molecule 1 as well. It is a membrane glycoprotein encoded by the Havcr1 gene. Expressed on various cells as a receptor for many viruses and ligands, TIM-1 has been reported to involve in kidney diseases, atopic diseases, T-cell activation and cancers.^2-4 Antibody–drug targeting TIM-1 has also been developed for treating cancers with high TIM-1 expression.⁴ In Phase I clinical trial, CDX-014, an antibody–drug conjugate against TIM-1, exhibited a manageable toxicity profile and early signs of activity in 16 patients with advanced refractory renal cell cancer.⁴ However, no scholar considers the role of TIM-1 on B cells in these studies, not to speak of as a B-cell-specific checkpoint molecule.

Immune checkpoints are host molecules that have the ability to suppress immune responses.⁵ Their expression will suppress antitumor immune responses in cancer. In past studies, researchers have primarily focused on enhancing the immune response of T cells, particularly cytotoxic T lymphocytes, in the fight against cancer. The discovery of immune checkpoints, such as the PD-1/PD-L1 and the CTLA4/B7 pathways sparked a wave of interest in T-cell-specific immune checkpoint inhibitors for cancer treatment. However, the authors of this study sought to investigate B cells, as they are one of the most abundant cell types that infiltrate tumors, especially in melanoma.

To understand the role of B cell subsets in melanoma, the researchers employed the well-established B16F10 melanoma mouse model and collected immune cells at 7, 10, and 16 days after tumor cell injection. Subsequently, bulk messenger RNA-sequencing was performed on the tumor group along with draining lymph nodes (dLNs), nondraining lymph nodes (ndLNs), and spleen groups at Day 16 postinjection to examine the expression profiles of B cells across different groups. To further comprehend the heterogeneity of B cells within tumor infiltration, single-cell RNA sequencing (scRNA-seq) combined with B-cell receptor sequencing analysis were utilized to identify expanding subsets of B cells within these three groups over time. Notably, a distinct cluster of proliferative germinal center-like B cells (cluster 3) was observed in dLNs that exhibited a corresponding increase in tumor growth. The researchers identified a key gene, Havcr1, encoding TIM-1 molecule, specifically expressed by cluster 3. Based on bulk RNA-seq and flow cytometry analysis, TIM-1⁺ B cells from dLNs and ndLNs showed increased proliferation and tended to differentiate into plasma cells compared with TIM-1⁻ B cells. Furthermore, TIM-1⁺ B cells expressed higher levels of various coinhibitory and immunoregulatory molecules, including PD-1, TIGIT, LAG3, TIM-3, CD39, CD73, and IL-10.

Subsequently, the researchers found that conditional deletion of Havcr1 gene in B cells significantly inhibited tumor growth in multiple melanoma models as well as other cancer models. However, genetic deletion of known checkpoints TIM-3, TIGIT, PD1, or LAG3 in B cells had little or no effect on tumor growth. In addition, deletion of Havcr1 in all T cells had no effect on tumor growth, demonstrating the vital role of TIM-1 specifically expressed on B cells. Therefore antitumor effect of anti-TIM-1 therapy was completely dependent on TIM-1 expression on B cells. Furthermore, they found that the TIM-1 blockade of B cells can also enhance the antitumor immune effect of PD-1 blocking therapy, providing a possible new choice for checkpoint combination therapy. Moreover, after analyzing public data of tumor-infiltrating leukocytes (TILs) in human tumors, Bod et al.¹ also found that higher expression of Havcr1 corresponded to poorer overall survival in lung, pancreatic and stomach adenocarcinoma patients. They identified a cluster of TIM-1⁺ B cells simultaneously expressing several know coinhibitory molecules. The signature of this subset of B cells is somehow similar to human exhausted T cells.

As summarized in Figure 1, the experimental results and analysis of scRNA-seq profile revealed several impacts of Havcr1 deficiency in B cells, which might explain why it can suppress tumor growth. First, T-cell receptor sequencing analysis revealed that deficiency of Havcr1 in B cells led to an increased immune cell infiltration and an increased antigen-specific CD8⁺ TILs to enhance effector T-cell responses, including stronger cytotoxicity of CD8⁺ T cells, higher CD107a expression, and more coexpression of granzyme B as well as perforin. Second, Havcr1 deficiency in B cells has minimal effect on the humoral immunity against tumors and lymphatic organs. However, it enhanced B-cell antigen presentation to CD4⁺ T cells, expanded CD4⁺ T helper cells, and reduced FOXP3⁺ T_reg cell expansion. Third, TIM-1-deficient B cells showed higher expression and sensitivity to type I and type II interferon.

In summary, this study challenges the previously controversial role of B cells in antitumor immunotherapy and paves the way for innovative therapeutic approaches. TIM-1 blockade therapy has shown early signs of effect in cancers with high TIM-1 expression in clinical trials.⁵ The significance of TIM-1 as a checkpoint molecule seems to lie in its expression on B cells rather than on T cells or cancer cells. By understanding this, researchers can expand TIM-1 blockade therapy to cancers with low or no TIM-1 expression. This breakthrough has the potential to revolutionize the design of immunotherapeutic strategies, opening up new avenues for treating cancer patients. However, further studies are warranted to explore which receptor makes TIM-1 the immune checkpoint on B cells. Additionally, further investigating the therapeutic potential TIM-1 blockade of B cells in preclinical and clinical settings could lead to the development of more novel treatments that harness the power of immune checkpoint on B cells in combating tumors.

Wenwen Liu: Writing—original draft (lead); writing—review and editing (equal). Jian Huang: Conceptualization (lead); funding acquisition (lead); supervision (lead); writing—review and editing (equal). Both authors have written and approved the article.

The authors declare no conflicts of interest.

Not applicable.