TMEPRE model predicts anti-PD1 treatment response
The model was developed using gene expression data of colorectal cancer patients and has two components: (28 genes, Supplementary table 1) and (29 genes, Supplementary table 2).
To date, most of available gene expression datasets of anti-PD1 response were performed using melanoma as the model system. TMEPRE was validated on 3 datasets of melanoma patients who received anti-PD1 treatment. In the first dataset, the survival analysis of the TMEPRE prediction model resulted in a significant hazard ratio (n=16, pretreatment samples, GSE78220, HR=4.59, p-value=0.056, Figure 1A). In the second dataset, although the p-value of the survival analysis of the TMEPRE prediction model is large (n=21, sampling before cycle 1 day 0, GSE91061, HR=2.12, p-value=0.115, Figure 1B), the separation of survival between TMEPRE predicted responder group and TMEPRE predicted nonresponder group is still clearly observed. In the third dataset, the survival analysis of the TMEPRE prediction model resulted in a significant hazard ratio (n=21, sampling at an early treatment time point before cycle 1 day 29, GSE91061, HR=5.04, p-value=0.003, Figure 1C). Data used to validate anti-PD1 response (GSE78220 and GSE91061) are from melanoma patients and on the Illumina HiSeq platform. Data used to train the model (GSE13294, GSE26682, GSE18088, GSE39084) are from colorectal cancer patients and on the Affymetrix platform. No anti-PD1 response data or survival data was used in the training of the TMEPRE model. Despite technical noise introduced by different cancer types and different data platforms, TMEPRE model showed predictive powers for responders to anti-PD1 treatment in all three validations.
The underlying biology of TMEPRE model measures amounts of tumor infiltrating CD8+ T cells and characters of tumor infiltrating terminally exhausted CD8+ T cells
In the dataset of all 454 samples, the countings of tumor infiltrating cytotoxic lymphocytes were read out using MCP-counter and TIDE cytotoxic T lymphocytes count.8,12 The first component of TMEPRE model, score, positively correlates with counting of MCP-counter cytotoxic lymphocytes (r=0.82, r.msi=0.81) and TIDE cytotoxic T lymphocytes (r=0.68, r.msi=0.83) (Figure 2). The relative ranges coverage of scores of MSS samples is larger than the relative range coverage of MCP-counter score and TIDE score(=0.89, =0.67, =0.81). These results suggested that in MSS colorectal tumors that harbor fewer tumor infiltrating immune cells, might be a more sensitive measurement of tumor infiltrating cytotoxic lymphocytes. The reason might be that is specifically designed for the tumor microenvironment of colorectal cancer, whereas the other methods like MCP-counter and TIDE are not.
The second component of TMEPRE model, score, is designed to measure whether tumor infiltrating CD8+ T cells can respond to anti-PD1 treatment. To test whether indeed capture this character of tumor infiltrating CD8+ T cells, we read out the scores of signature in two subgroups of dysfunction CD8+ T cells isolated from tumors and chronic viral infection: terminally exhausted tumor infiltrating CD8+ T cells that can no longer respond to anti-PD-1 therapy and progenitor exhausted tumor infiltrating CD8+ T cells that can still respond to anti-PD-1 therapy (GSE122713).13 Because signature is derived from gene expression data of bulk tumor sample, the source of gene expressions originates from a mixture CD8+ T cells, tumor cells and other tumor infiltrating immune cell types in the tumor microenvironment, while the progenitor/terminal exhausted tumor infiltrating CD8+ T cells data are generated from isolated CD8+ T cells. Therefore, when scores were read out, only genes in that primarily originated from CD8+ T cells are used. For each of 29 genes in , median expression values of 16 purified main immune cell types were compared using BloodSpot with HemaExporer human hematopoiesis database.14 A gene is considered as mainly expressed by CD8+ T cells when CD8+ T cell is among the top 2 immune cell types expressing this gene. Seven genes (CCL5, CD2, CD48, CD84, FAM78A, HCST, IL21R) in passed these criteria and two genes in are inhibitor receptors on CD8+ T cells used to define early terminal exhausted CD8+ T cells (HAVCR2, PDCD1). These nine genes were used to read out scores in the isolated progenitor/terminal exhausted tumor infiltrating CD8+ T cells dataset. In both tumors and chronic viral infection, the score of are significantly higher in the subgroup of progenitor exhausted tumor infiltrating CD8+ T cells ( <0.001, = 0.048, Figure 3). Therefore, the score of indeed captures the characters of progenitor exhausted tumor infiltrating CD8+ T cells that can still respond to anti-PD1.
Global pattern of TMEPRE model in MSI and MSS colorectal tumors
The TMEPRE model was read out in 454 colorectal samples (MSI=131, MSI-L=23, MSS=284, Unknown=16). A splitted heatmap was plotted.15 Tumors displaying a pattern of sufficient CD8+ T cell infiltration but no pattern of CD8+ T cell terminal exhaustion are considered as potential responders to anti-PD1 therapy (Figure 4).
Within 284 MSS tumor samples, 10.6% (n=30) are classified as responders and 89.4% (n=254) as nonresponders. Among MSS nonresponders, 86.6% (n=246) showed no sufficient tumor infiltrating CD8+ T cell, 2.8% (n=8) showed sufficient tumor infiltrating CD8+ T cell but those CD8+ T cells display patterns of terminally exhausted CD8+ T cell. As expected, the anti-PD1 resistance mechanism of the majority of MSS tumors is no sufficient amount of tumor infiltrating CD8+ T cells.
Within 131 MSI tumor samples, 67.2% (n=88) are classified as responders and 32.8% (n=43) as nonresponders. Among the MSI nonresponders, 16.0% (n=21) showed no sufficient tumor infiltrating CD8+ T cell, 16.8% (n=22) showed sufficient tumor infiltrating CD8+ T cell but those CD8+ T cells display patterns of terminally exhausted CD8+ T cells. Therefore, approximately 50% of MSI nonresponders are caused by terminal exhaustion of CD8+ T cells in the tumor microenvironment, and the rest 50% of MSI nonresponders are caused by no sufficient amount of tumor infiltrating CD8+ T cells.
TMEPRE model identified 10.6% of MSS and 67.2% of MSI colorectal cancer patients whose tumors show biological characters that can potentially benefit from anti-PD1 treatment. These predicted percentages of responders in MSS tumors and MSI tumors are consistent with the reported benefit of immune related disease control rate at 20 weeks of a cohort of colorectal cancer patients treated with pembrolizumab.16