In this study, we evaluated the presence of immune-specific exosomal miRNAs in the plasma of patients with mRCC prior to initiation of ICI. We found significantly lower relative levels of miR155 in responders to ICI, when compared to non-responders. Our findings are in keeping with what has been reported in plasma of patients with melanoma, wherein higher levels of miR155 were seen in patients, as compared to healthy controls, and lower levels of miR155 in patients was associated with improved clinical outcomes21,22.
While our results are hypothesis-generating in nature, there is biological rationale for how miR155 could serve as a predictive biomarker of the antitumour immune response. MiR155 is the product of the MIRHG155 gene, also referred to as the B-cell Integration Cluster (BIC) gene and has often been referred to as the “master regulator of inflammation”, owing to its role in modulating the inflammatory response in autoimmune conditions, and oncogenesis. Its most well-characterized role is in negative regulation of activation induced cytidine deaminase (AID), an enzyme that is critical for diversification of the antibody repertoire23.
While most of our understanding of the anti-tumour immune response has focused on T-cells, emerging evidence supports an important role of B-cells in this complex process24. B-cells are those lymphocytes which express surface immunoglobulins and ultimately lead to antibody production. Immature B-cells initially develop in the bone marrow where they undergo V(D)J recombination, before differentiating in germinal centres (GC) of secondary lymphoid structures (spleen, lymph nodes). During B-cell differentiation, several processes take place which allow for diversification of the antibody repertoire. These include class switch recombination (CSR) and somatic hypermutation (SHM). The process of SHM causes point mutations in the variable region of an immunoglobulin, ultimately resulting in affinity maturation whereby the immunoglobulin’s affinity for an antigen is increased. Then, CSR takes place, resulting in expression of different antibody isotypes (such as IgM, IgG, IgE). The consequence of these processes is a rich antibody repertoire which can detect various antigens.
The critical interaction of B and T-cells is also important for achieving tumour control25. An important role of B-cells is the maintenance of tertiary lymphoid structures (TLS) in the TME. These are collections of B-cells, T-cells, and dendritic cells, which develop due to chronic inflammation. They function similarly to other lymphoid structures in that they allow for maturation and differentiation of B and T cells in areas adjacent to tumour beds, while also facilitating tumour antigen presentation, and infiltration of lymphocytes within tumours26. These TLS have been demonstrated to have prognostic implications in NSCLC, breast, and ovarian cancers27–29. Importantly, in RCC, TLS have been demonstrated to produce high levels of IgG, which become bound to tumour cells; the presence of these IgG-bound tumour cells in the TME has been and associated with a higher response rate to ICI30. Saul and colleagues have demonstrated that in melanoma tissue, not only are unique IgG repertoire subclasses present, but also higher AID mRNA expression is seen within in that tissue suggesting an important role for humoral immune response in TLS31. Given our findings, we hypothesized that lower levels of miR155 in R resulted in relatively higher AID activity, and thus a more diverse antibody repertoire against tumour antigens leading to a more robust anti-tumour immune response.
MicroRNA-155 has multiple regulatory functions and its exact role in RCC is not clear. Lower tissue expression of miR155 has been reported as a poor prognostic biomarker in RCC32. On the other hand, miR155 levels have been shown to inversely correlate with VHL expression, such that higher levels of miR155 lead to lower VHL expression and thus promotion of angiogenesis and tumorigenesis33. Taken together, these findings emphasize that miR155 is highly relevant in RCC, but further work is needed to clarify its exact role in this malignancy.
Our study has several limitations, and the results will need to be validated. First, the small sample size is a limiting factor in being able to make a conclusion that miR155 is in fact a predictive biomarker of response to ICI in mRCC. Validation studies in larger cohorts would be valuable to confirm our finding. Furthermore, miR155 is not specific to RCC and is expressed in other malignancies, inflammatory conditions, and normal tissue as well. We overcame this limitation by selecting patients and healthy control participants who did not have a history of other malignancy or autoimmune conditions. The retrospective nature of our study is a further limitation and as previously mentioned, prospective evaluation in a larger cohort of patients is needed. This is particularly important as treatment may alter the presence of tumour neoantigens, and thus impact the humoral immune response.
Our study adds to a growing body of evidence that is exploring the humoral immune system in the anti-tumour immune response. As questions rise regarding resistance to ICI and there are expanding indications of these drugs not only in advanced disease but also in the adjuvant setting, exosomal miRNAs hold promise as potential biomarkers of response to immunotherapy in RCC and should be explored further.