PREreview of Intratumoral mregDC and CXCL13 T helper niches enable local differentiation of CD8 T cells following PD-1 blockade

Review of: Intratumoral mregDC and CXCL13 T helper niches enable local differentiation of CD8 T cells following PD-1 blockade.” BioRxiv. DOI: https://doi.org/10.1101/2022.06.22.497216

This preview was written by Yip Ming Tsun Sophronia , a trainee in the Rio Sugimura lab at the University of Hong Kong as a part of her training in studying cancer immunotherapy.

Summary:

To investigate the detailed molecular and cellular mechanisms involved in triggering an effective anti-tumor response after PD-1/PD-L1 blockade in tumors, this paper study the TME composition of surgically-resected tumor lesions from HCC patients treated with neoadjuvant PD-1 blockade. They compared the TME composition between T cell-rich non-responder patients and T cell-rich responder patients. By using scRNA-seq and single-cell T cell receptor sequencing (scTCRseq), they found out that PD-1high effector-like CD8 T cells and CXCL13+ T helper cells were clonally expanded in tumors of responders when compared to non-responders, while PD-1high terminally exhausted CD8 T cells and Tregs were clonally expanded preferentially in tumors of non-responders. They later identify a cellular triad composed of mregDC, PD-1high  CD8 progenitor T cells, and CXCL13+ T helper cells in tumors of responders using multiplex imaging analysis and proposed possible interactions between the triad using receptor-ligand mapping, concluding that these interactions drive local differentiation of PD-1high CD8 progenitor T cells into effector like phenotype instead of terminally exhausted phenotype upon PD-1 blockade. 

Major comments

Strengths:

1. New perspective on the role of mregDC in PD-1 blockade responsiveness

-In the past, the responsiveness of PD-1 blockade therapy is often determined by the abundance of T cells in tumor sites but emerging evidence also suggested that a proportion of non-responders are also rich in T cells in tumor sites. This paper showed a comprehensive comparison between T cell-rich responders and non-responders. They proposed a novel mechanism of the role of mregDC and its interactions with a specific subset of T cells attributed to the anti-tumor effect after PD-1 blockade. This finding could target molecules to enhance mregDC interaction with the T cells in the triad that could enhance the responsiveness of PD-1 blockade therapy in patients.

2. Multiple angles to prove that PD-1high CD8 T cells and CXCL13+ Th are important in determining the responsiveness of PD-1 blockade

-The authors conducted multiple experiments to support this claim, firstly scRNA-seq to show that PD-1high CD8 T cells and CXCL13+ Th were enriched in the tumour site of responders when compared to non-responders. They also used scTCRseq and compared the quantity of T cell clones before and after treatment to show that they expand locally and clonally upon PD-1 treatment. Apart from quantitative results, BaseScope TCR imaging analysis showed the spatial expansion of PD-1high CD8 T cells and CXCL13+ Th on the sample.

Moreover, the author also showed a correlation between the cellular abundances of CXCL13+ Th and PD-1high effector-like CD8 T cells, evidence for the later discussion of the possible positive regulation between these 2 cell types in the cellular triad. These data provide solid evidence that these 2 groups of T cells play a significant role in PD-1 blockade responsiveness.

To be improved:

1. Missing link between how PD-1 blockade affects mregDC phenotype and its interactions with CXCL13+ Th, and PD-1hi CD8 T cells.

-They proposed possible interactions between mregDC, CXCL13+ Th, and PD-1hi CD8 T cells by receptor-ligand mapping and how their interactions lead to a favorable response in patients. However, they did not show why PD-1 blockade in responders enhances the formation of the triad and their interactions.

2. Direct evidence of mregDC and CXCL13 T helper niches enable local differentiation of CD8 T cells.

-Although they conclude that the interaction between mregDC and CXCL13 T helper enhances the differentiation of PD-1hi CD8 T progenitors to an effector, direct evidence is missing. They could validate that with co-culture experiments and perform transcriptome analysis on CD8 T cells, to see whether the differentiation to effector phenotype is enhanced in the presence of mregDC and CXCL13 Th cells.

3. Which specific receptor-ligand interactions are enhanced after PD-1 blockade?

-They used receptor-ligand mapping to identify the expression of candidate molecules that might promote interactions between triads to account for the enhanced anti-tumor effect after PD-1 blockade. They proposed the possible role of CH25H, LTB, CD40L, and XCL1 as causative factors in the triad. The inhibitors of these molecules could validate their roles in a co-culture setting.

Minor comments  

-Why are B cells also enriched within the triad?

-Co-culture experiments to dissect the molecular mechanisms of how the cellular triad interacts with each other to support the hypothesis in the discussion part

Overall:

The former part of the paper show really convincing results of the enrichment of PD-1high CD8 T cells and CXCL13+ Th within tumor sites in responders, but mechanisms on the cellular triad could be improved. Relating how PD-1 blockade leads to the formation of the triad and their interactions, and identification of causative factors that account for the enhanced anti-tumor effect will enhance the impact of the manuscript.