We are grateful for the thoughtful and constructive feedback from Sun et al. 1 regarding our recent study on the immunologic dynamics and clinical outcomes associated with sequential treatment with ramucirumab plus docetaxel after PD-1 blockade in advanced non-small cell lung cancer 2. Their letter raises important questions about the interpretation of anti-PD-1 antibody-bound T cells and the broader challenges of relating peripheral immune measurements to the functional consequences of checkpoint blockade. We appreciate the opportunity to clarify the rationale and scope of our analysis. First, Sun et al. emphasize that IgG4 positivity reflects antibody binding rather than providing a direct measure of functional PD-1 pathway blockade. We agree that detecting IgG4 on T cells does not itself define receptor occupancy thresholds or downstream inhibition of PD-1 signaling. Our aim was not to claim that IgG4 positivity directly quantifies functional PD-1 blockade but to monitor the dynamics of antibody-bound T-cell populations during subsequent systemic therapy. In our previous work, we demonstrated the methodological and immunological relevance of detecting antibody-bound T cells using anti-IgG4 staining 3. The current study builds on this foundation by examining the longitudinal changes in these antibody-bound T-cell subsets and their associations with clinical outcomes during subsequent treatment. We also agree with Sun et al. that antibody binding on circulating T cells can be influenced by various factors. In fact, our previous data showed that antibody binding on T cells does not necessarily correlate with plasma nivolumab concentrations 3. Therefore, the main focus of the current study was to identify peripheral T cell subsets retaining antibody binding and to analyze their longitudinal changes within the same patients during subsequent therapy after immune checkpoint inhibitor treatment. Regarding pembrolizumab, we have previously reported that the temporal dynamics of antibody binding on T cells were generally similar to those observed with nivolumab 4. Considering these factors, we adopted a pooled exploratory analysis in the present study. Second, Sun et al. appropriately note that peripheral immune dynamics do not necessarily reflect immune conditions within the tumor microenvironment. Circulating immune populations represent a dynamic compartment influenced by cellular trafficking, tissue redistribution, and treatment-related perturbations. Our study was designed as a prospective clinical immune-monitoring analysis based on peripheral blood sampling, which enables repeated longitudinal assessment with minimal invasiveness. We agree that integrating tumor-level analyses would provide valuable mechanistic insights. These analyses could include the characterization of tumor-infiltrating lymphocytes, assessment of clonal relationships between tumor and circulating T cells, and functional profiling of regulatory T cell phenotypes. Such analyses were beyond the scope of the present clinical study but represent important directions for future investigation. Third, Sun et al. highlight the potential influence of treatment-related factors such as docetaxel-associated cytotoxicity, granulocyte colony-stimulating factor (G-CSF) administration, and inflammatory events. We agree that these factors are relevant when interpreting systemic immune measurements during cytotoxic chemotherapy. In our multicenter prospective cohort study, we predefined peripheral immune sampling time points relative to treatment initiation. This allowed us to conduct longitudinal assessments within a consistent clinical framework. Prophylactic G-CSF was used in most patients (25 out of 27) according to routine institutional practice. However, the study was not specifically designed to adjust for all potential time-varying clinical modifiers, and residual confounding cannot be excluded. Future analyses incorporating more detailed data on treatment exposure, inflammatory events, and supportive care would help refine the interpretation of peripheral immune dynamics. Taken together, our aim was to explore whether longitudinal changes in antibody-bound T-cell subsets after PD-1 blockade were associated with subsequent treatment outcomes. In our cohort, such changes were associated with clinical outcomes following sequential ramucirumab plus docetaxel therapy, suggesting the potential clinical relevance of monitoring these immune populations. In summary, we appreciate the insightful perspectives provided by Sun et al., which highlight important methodological considerations for interpreting antibody-bound T-cell measurements. They emphasize the importance of integrating peripheral immune monitoring with more detailed mechanistic studies of checkpoint engagement and tumor immune biology. We hope that our findings will contribute to further investigation of peripheral immune correlates in this setting. Kinnosuke Matsumoto: conceptualization, writing – original draft. Yujiro Naito: conceptualization, writing – original draft, writing – review and editing. Takayuki Shiroyama: conceptualization, writing – review and editing, supervision. This research was partly supported by the Japan Science and Technology Agency Support for Pioneering Research Initiated by the Next Generation (JST SPRING) (JPMJSP2138), the Japan Agency for Medical Research and Development (AMED) under grant number (25ama221533h0002), the Japan Society for the Promotion of Science (JSPS) Grants-in-Aid for Scientific Research (S) (20H05698), the Ministry of Education, Culture, Sports, Science and Technology (MEXT) under grant number (JPMXP1323015484), and Eli Lilly Japan. The authors have nothing to report. The authors have nothing to report. Kinnosuke Matsumoto reports receiving grants from Eli Lilly Japan and honoraria for lectures from Bristol-Myers Squibb, Chugai Pharmaceutical, Eli Lilly Japan, Merck Sharp & Dohme, and Ono Pharmaceutical outside of the submitted work. The other authors declare no conflicts of interest. This article is linked to Matsumoto et al. papers. To view these articles, visit https://doi.org/10.1111/cas.70336 and https://doi.org/10.1111/cas.70379. Data sharing not applicable to this article as no datasets were generated or analysed during the current study.
Matsumoto et al. (Mon,) studied this question.