Exploration of different immunogenomic TMEs in gastric cancer based on HER2 and PD-L1 expression.
Authors: Gu Do-eon , Kwon Woo Sun , Lee Choong-kun , Kim Hyunki , Rha Sun Young 
Published: July 2024

e16073 Background: Recent results from the KEYNOTE-811 study showed that first-line pembrolizumab was superior to placebo with respect to progression-free survival (PFS) in combination with trastuzumab and chemotherapy in patients with HER2 positive metastatic gastric cancer (GC). Interestingly, In the subgroup analysis, the PFS benefit with pembrolizumab was significant in patients with tumors with PD-L1 CPS ≥1, whereas there was no benefit in the small subgroup of patients with tumors with CPS < 1. Therefore, this study is to investigate immune-genomic differences in the tumor microenvironment according to the composition of surrounding cells and the expression status of HER2 and PD-L1. Methods: Utilizing single cell RNA sequencing (scRNA seq) and TCR sequencing, we examined 14 HER2 positive and 18 negative primary GC samples with CPS score based on PD-L1 IHC 22C3 pharmDx. Among the HER2-positive group, there were 4 samples in the CPS < 1 (negative) and 10 samples in the CPS ≥1 (positive) group. In the HER2 negative group, there were 6 samples in the CPS < 1 and 12 samples in the CPS ≥1. Results: First, when analyzing the distribution of total cells according to HER2 status, in the HER2 positive group, more endothelial cells ( p= 0.034) and fibroblasts ( p= 0.054) appeared in the PD-L1 negative group compared to the PD-L1 positive group, while there was no difference in the HER2 negative group. Furthermore, Through TCR analysis, the exhausted and cytotoxic score exhibit contrasting trends between HER2 negative and positive groups. In HER2-positive patients, the exhausted and cytotoxic score appear significantly higher in the CPS negative group. Next, we evaluated the proportion of immune cells according to HER2 status. The observed of CD4 Treg ( p= 0.084) and CD8 exhausted T (Tex), ( p= 0.036) cells was increased in the HER2 positive compared to the negative. When regrouped according to PD-L1 expression, only the HER2 positive group showed a tendency to increase the observation of CD4 Treg and CD8 Tex cells compared to positive in PD-L1 negative. In addition, we investigated whether the expression pattern of other immune checkpoint molecules differed depending on the PD-L1 expression pattern in both HER2 positive and negative patients. In the HER2 negative group, the expression of immune checkpoint proteins was similar regardless of the CPS expression group, or the expression of some proteins was higher in the CPS positive group. In other hands, in HER2 positive, CPS < 1 group had higher expression of immune checkpoint proteins, such as PDCD1, LAG2, BTLA, VISTA and TIGHT. Conclusions: The high proportion of CD4 Tregs, CD8 Tex and the elevated exhausted score in the subgroup with CPS < 1 in HER2 positive GCs suggesting immune exclusive environment compared to her-2 and PD-L1 coexpressing group. Considering increased expression of immune checkpoint proteins other than PD-1/PD-L1, for patients with HER2-positive and CPS < 1, combination therapy with other immune checkpoint inhibitors rather than anti-PD-1/PD-L1 may be suggested.