A lung squamous cell carcinoma demonstrating spontaneous partial regression of the primary tumor with lymph node metastasis: a case report with analysis of the tumor immune microenvironment

Introduction

Spontaneous regression (SR) of malignant tumors, defined as a partial or complete tumor reduction in the absence of effective therapy, occurs in approximately 1 in 60,000–100,000 cancer cases (1,2). SR has been observed in several tumor types but is relatively rare in lung cancer, accounting for about 5% of reported cases in the literature (1,3). Because SR occurs frequently following acute infection, vaccination, or chemotherapy, it has long been considered that antitumor immune responses by host T lymphocytes reacting against tumor cells are associated with SR (4,5). Accordingly, understanding the mechanisms underlying SR may provide important clues for elucidating the therapeutic effects of immunotherapy. In non-small cell lung cancer (NSCLC), SR has also been reported in the absence of apparent triggering factors; however, only a limited number of studies have directly examined the underlying immune context at the tissue level (6-8). Here, we report a lung squamous cell carcinoma case that was difficult to diagnose by biopsy. The patient exhibited partial SR of the primary lung tumor during observation, subsequently developed mediastinal lymph node (LN) metastasis, and was treated surgically. A detailed pathological analysis of the resected samples was performed to elucidate the mechanisms of SR. We present this article in accordance with the CARE reporting checklist (available at https://tlcr.amegroups.com/article/view/10.21037/tlcr-2025-1-1466/rc).

Case presentation

A 74-year-old Japanese man was referred to our hospital for further lung nodule evaluation. Forty-one days before his visit (day 0), chest computed tomography (CT) performed during the workup for chest pain revealed a solid, irregularly shaped nodule in the right lower lobe, measuring 2.4 cm in diameter (Figure 1A). A repeat CT scan on day 15 demonstrated slight lesion enlargement. No hilar or mediastinal lymphadenopathies were observed. He did not have any symptoms, and no particular physical findings were noted. The patient had a history of heavy smoking (60 pack-years). He was receiving medication for diabetes mellitus and had no history of malignant tumor. There was no history of infectious diseases or vaccinations during the observation period.

Figure 1 Radiological findings. (A) Serial chest CT images with schematic of lesion locations and slice levels. Day 0: initial CT showed a 2.4 cm × 1.6 cm pulmonary opacity in the right lower lobe; the superior component appeared nodular (arrowheads). Day 83: the primary lesion began to shrink. New enlargement of the subcarinal (#7) and interlobar (#11i) LNs was observed (arrowheads). Day 124: the primary lesion further decreased, whereas both LNs increased. Day 197: the primary lesion decreased to 1.6 cm × 0.7 cm; the #7 LN continued to enlarge, while the #11i LN decreased. Primary, primary lung lesion; Met #7 LN, metastatic #7 LN; #11 LN: nonmetastatic #11i LN; main, main component; sup, superior adjacent component. (B) Positron emission tomography-CT of each lesion. On day 104, increased ¹⁸F-fluorodeoxyglucose uptake was observed in the primary lesion (SUVmax 3.9→4.6), the #7 LN (9.6→12.3), and the #11i LN (11.7→12.9). (C) Change in lesion size over time. Values represent changes from baseline (day 0). “Primary” denotes the mean of the maximum diameters of the main and superior components; LN size is the maximum short-axis diameter. CT, computed tomography; LN, lymph node; SUVmax, maximum standardized uptake value.

An endobronchial ultrasound-guided sheath biopsy was performed on day 65; however, the results suggested insufficient sampling. A follow-up CT performed on day 83 demonstrated new hilar lymphadenopathy, whereas the pulmonary nodule exhibited further partial regression. At that time, because malignancy could not be confirmed and the lesion showed shrinkage, surgery was not undertaken in accordance with the patient’s preference, and radiological surveillance with additional diagnostic evaluation was continued. On day 135, a CT-guided needle biopsy revealed fibrotic scarring and chronic inflammation in the lung nodule. Positron emission tomography-CT demonstrated persistent right #7 (subcarinal) and #11i (interlobar) LN enlargement with increased ¹⁸F-fluorodeoxyglucose uptake [maximum standardized uptake value (SUVmax) 12.3 and 12.9, respectively; Figure 1B]. Additionally, the primary lesion showed abnormal uptake (SUVmax 4.6) despite further size reduction. Endobronchial ultrasound-guided transbronchial needle aspiration of the right #7 node was performed; cytology suggested squamous cell carcinoma but was inconclusive. On day 197, follow-up CT demonstrated persistent #7 LN enlargement, while the pulmonary nodule continued to shrink and the #11i LN showed a marked reduction (Figure 1A,1C). Temporal changes in lesion size and the timing of diagnostic procedures are summarized in Table 1.

Given the progressive enlargement of the #7 LN, which was suspicious for malignancy, surgical intervention for definitive diagnosis and treatment was considered necessary, and surgery was performed on day 203 with the patient’s consent. Intraoperative assessment of the lung nodule and #7 LN revealed squamous cell carcinoma with nodal metastasis. Right lower lobectomy with level 2a-2 systematic LN dissection (ND2a-2) was performed. He was discharged without any postoperative complications. Histopathological examination confirmed that the 10-mm primary nodule and LN metastases were composed of non-keratinizing invasive squamous cell carcinoma (Figure 2A,2B). In the primary lung nodule, 70% of the area was replaced by fibrosis, with viable carcinoma cells accounting for the remaining 30%. LN metastasis was limited to station 7, and the final pathological stage was pT1aN2aM0 and pStage IIB. Adjuvant chemotherapy with cisplatin and vinorelbine was administered, but discontinued due to fatigue and visual disorder. He is under careful follow-up, and no recurrence has been detected.

Figure 2 Pathological examination. (A) H&E staining at low power of the primary lung lesion and the metastatic #7 LN. Boxes indicate areas displayed at higher magnification. (B) High-power H&E images of the tumor regions in the primary and metastatic lesions. (C) H&E and IHC staining for CD3, CD8, and CD163 within the fibrotic stroma of the primary lung lesion are shown. H&E, hematoxylin and eosin; IHC, immunohistochemistry; LN, lymph node.

To investigate the immunological features of each lesion potentially associated with SR, we performed additional immunohistochemical analyses. Inflammatory cells, including CD3⁺ and CD8⁺ T cells and CD163⁺ macrophages, were abundant in the fibrotic stroma of the primary lung lesion (Figure 2C). Programmed death-ligand 1 (PD-L1) expression was examined, as shown in Figure 3A. Human leukocyte antigen (HLA) class I expression was downregulated in both the primary and metastatic tumor cells. β2-microglobulin (B2M) and HLA-DR were positive in the primary tumor but lost in the metastasis. PD-L1 was diffusely positive with weak-to-moderate intensity in the primary tumor, but nearly negative in the metastasis.

Figure 3 Additional pathological examination. (A) IHC for antigen-presentation molecules and PD-L1 in tumor cells. (B) IHC for the indicated markers in tumor-infiltrating lymphocytes and macrophages within tumor nests. (C) Pseudocolored multiplex IHC images: hematoxylin (blue), CD8 (magenta), CD103 (orange), FOXP3 (green), CD163 (yellow). Multiplex IHC was performed using a destaining/antibody stripping/restaining workflow, and the results were pseudocolored in HALO. (D) Image-based cytometry of CD8 and CD103 staining intensities in tumor nests and stroma of the primary lung lesion. Processing (pseudocolor, segmentation, intensity) was done in HALO (color deconvolution). Quadrant intensity thresholds were defined by a pathologist, based on cell-level positive/negative criteria derived from reference microscopic fields. B2M, β2-microglobulin; FOXP3, forkhead box P3; HLA, human leukocyte antigen; ICOS, inducible T-cell costimulator; IHC, immunohistochemistry; LN, lymph node; PD-L1, programmed death-ligand 1.

Immune cell infiltration within the tumor nests evaluated using immunohistochemistry (IHC; Figure 3B) together with the corresponding cell density quantification (Table 2) revealed differences between the primary and metastatic lesions. CD8⁺ and CD103⁺ tumor-infiltrating lymphocytes (TILs) were more abundant in the primary lesion than in the metastatic lesion. Inducible T-cell costimulator (ICOS)-positive and forkhead box P3 (FOXP3)-positive TILs were relatively few, with no apparent difference observed between the lesions. CD163⁺ macrophages were sparsely observed within primary tumor nests and were even less frequent in the metastatic lesion. Overall, most immune cells were predominantly observed in stromal areas, whereas CD103⁺ cells showed a preferential enrichment within tumor nests. Multiplex IHC using a previously reported protocol (9) illustrated immune cell distribution in the primary tumor and the metastatic lesion (Figure 3C). Image-based cytometry of the primary lung lesion demonstrated CD103 enrichment within CD8⁺ T cells in tumor nests compared to that in the stroma (Figure 3D).

To quantitatively compare intratumoral immune cell infiltration, cell densities were measured within tumor nests. The two lesions were comparable as both exhibited tumor-nest-based structures of squamous cell carcinoma; fibrotic areas in the primary lesion and lymphoid follicles or sinus regions in the metastatic LN were not part of the tumor nests. Analyses were performed on digitized images using HALO software (version 4.0.5107.318; Indica Labs, Albuquerque, NM, USA), with morphologically necrotic or hemorrhagic areas excluded. Nuclear detection and chromogen positivity thresholds were optimized to minimize background, and cell segmentation was performed using a predefined cytoplasmic radius. Parameters were optimized on representative regions by H.Y., independently verified by Y.K., and uniformly applied to all regions of interest. Cell counting was conducted in five randomly selected, non-overlapping areas (0.08 mm²) per slide. For image-based cytometry of CD8 and CD103, regions of interest were manually annotated in tumor nests and adjacent stroma, yielding 4,432 and 1,632 analyzed cells, respectively. In the above staining procedures, the following primary antibodies were used: anti-HLA class I (clone EMR8-5; Medical & Biological Laboratories, Tokyo, Japan), anti-B2M (clone G-10; Santa Cruz Biotechnology, Dallas, TX, USA), anti-HLA-DR (clone TAL.1B5; Dako, Agilent Technologies, Glostrup, Denmark), anti-PD-L1 (clone 22C3; Dako), anti-CD3 (clone SP7; Nichirei, Tokyo, Japan), anti-CD8 (clone C8/144B; Nichirei), anti-CD103 (clone EPR4166(2); Abcam, Cambridge, UK), anti-ICOS (clone D1K2T; Cell Signaling Technology, Danvers, MA, USA), anti-FOXP3 (clone 236A/E7; Abcam), and anti-CD163 (clone 10D6; Novocastra, Newcastle, UK).

Ethical consideration

All procedures performed in this study were in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Ethics Committee of Kumamoto University (Genome-402). Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Discussion

This is the first case report to analyze, from an immunological perspective, different tumor developments in untreated NSCLC, characterized by SR of the primary tumor and enlargement of the mediastinal LN. In this case, immune cells, including infiltrating T cells, were generally abundant in the primary tumor but decreased in the metastasis. The contrasting immune cell conditions of the primary tumor being “hot” and the metastasis “cold” were consistent with the antigen presentation profile. A “cold” tumor immune microenvironment has been described in lung and breast cancer metastases in settings unrelated to SR; however, this pattern has not been consistently demonstrated in LN metastases (10,11). Although Ichiki et al. reported increased intratumoral CD103⁺ TILs in LN metastases of lung squamous cell carcinoma, the biological significance of this observation remains unclear (12). To our knowledge, the present report is the first to describe, based on direct immune profiling, that LN metastasis can exhibit a more immunologically “cold” phenotype than the corresponding primary tumor.

As originally proposed by Everson and Cole, SR refers to the partial or complete disappearance of a malignant tumor in the absence of effective anticancer therapy (1). In the present case, the primary lesion demonstrated radiological shrinkage prior to surgical resection without any local or systemic treatment and was therefore described as partial SR. However, radiological regression does not necessarily indicate a true reduction in viable tumor cells, which represents an important limitation of the present observation. Although non-specific inflammatory changes followed by fibrotic contraction can potentially lead to lesion shrinkage, this mechanism was considered unlikely in the present case, as no bronchovascular convergence was identified on CT during the regression phase. The extensive fibrosis and lymphocytic aggregation observed histologically in the primary lesion were more consistent with tumor cell loss and subsequent tissue remodeling.

Immunological modulation has been considered an important mechanism of SR in NSCLC and SCLC, as in other cancer types (13-15). Some previous studies have reported that CD8⁺ and/or CD4⁺ TILs were identified in the NSCLC specimens that showed SR (6,7). Beyond previous reports of CD8⁺ T-cell infiltration in NSCLC specimens with SR, this case enabled a direct comparison between the regressed primary tumor and its progressive metastatic lesion, revealing a relative reduction of CD8⁺ TILs in the latter. This contrast is suggestive of a potential role of CD8⁺ T cells in SR, although causality cannot be inferred from a single case.

In the present case, no apparent background factors such as infection, vaccination, chemotherapy, immunomodulatory medications, or local treatment to the regressed lesion were identified. At least 25 NSCLC cases of SR without such triggers have been reported, including 15 cases summarized in the comprehensive SR review by Walls et al. and 10 additional cases identified through our own literature search (6-8,16-23). Among these cases, squamous cell carcinoma was the most frequent histological subtype (12 cases), and in four cases, progression of other lesions after SR was observed, partially overlapping with the clinical features of the present case. Notably, with one exception, most reported cases underwent biopsy, mainly bronchoscopic or percutaneous needle biopsy, prior to SR, typically within three months. Biopsy-related tumor injury has been proposed as a potential trigger for antitumor immune response through tumor antigen release, generation of danger signals, and cytokine production, which may contribute to early steps of the cancer immunity cycle (24,25). In the present case, the initial bronchoscopic examination was nondiagnostic, making substantial tumor injury unlikely; however, biopsy-related T-cell responses cannot be entirely excluded. Among the 25 cases described above, only three reported T-cell infiltration assessed by IHC, and to our knowledge, no prior study has directly compared immune features between regressed and non-regressed lesions within the same patient (6-8). Further accumulation of immunological data in SR cases is therefore warranted to clarify the underlying mechanisms.

In this study, loss of B2M and HLA-DR was observed in the enlarged metastatic lesion. Consistent with our findings, Datar et al. demonstrated that NSCLC with downregulated B2M subunit expression showed fewer infiltrating CD4⁺, CD8⁺, and FOXP3⁺ T cells, and similar findings were observed in the NSCLC cohort with reduced HLA-DR expression (26). Additionally, reduced expression of B2M or HLA class II subunits was associated with poor prognosis, increasing the risk of death with a hazard ratio of 2–3 (26). Thus, loss of B2M and HLA-DR in the metastatic lesion may reflect immune-escape-related alterations described in previous studies and may partly explain the progression of the metastatic lesion, in contrast to regression of the primary tumor in this case.

Besides CD8⁺ cells, integrin αE subunit (CD103)-positive cells infiltrated the tumor nests more densely than did the stroma at the primary site, but this pattern was not clearly observed in the metastatic lesion. CD103 is a canonical marker of cytotoxic CD8⁺ tissue-resident memory T cells (TRMs), and CD103⁺ TRMs not only inhibit tumor growth via granzyme B and interferon gamma but have also been associated with a favorable prognosis (27). Indeed, Gauthier et al. reported that inhibiting CD103 signaling in CD8⁺CD103⁺ T cells in a tumor-slice co-culture reduced their recruitment to epithelial areas and diminished cytolytic activity (28). These CD103-mediated mechanisms may be consistent with the intratumoral enrichment of CD103⁺ cells observed in the SR lesions.

Conclusions

This case suggests that a contrasting immune cell environment, characterized by a “hot” primary lesion and a “cold” LN metastasis, may contribute to primary tumor SR and metastatic disease progression.

Acknowledgments

We would like to thank K.I. Stainer Inc. (Kumamoto, Japan) for their technical assistance and Editage (www.editage.jp) for English language editing.

Footnote

Reporting Checklist: The authors have completed the CARE reporting checklist. Available at https://tlcr.amegroups.com/article/view/10.21037/tlcr-2025-1-1466/rc

Peer Review File: Available at https://tlcr.amegroups.com/article/view/10.21037/tlcr-2025-1-1466/prf

Funding: This work was supported by a grant from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (No. 20H03459, to Y. Komohara) and a grant-in-aid for scientific research from the Japan Society for the Promotion of Science (No. 25K18903, to T. Koga).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://tlcr.amegroups.com/article/view/10.21037/tlcr-2025-1-1466/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All procedures performed in this study were in accordance with the ethical standards of the institutional and/or national research committee(s) and with the Declaration of Helsinki and its subsequent amendments. Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the editorial office of this journal.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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