Is immune effector cell-associated enterocolitis a CAR T-cell lymphoproliferative disorder?

Dear Editor,

We have read the recent article titled “Immune Effector Cell-Associated Enterocolitis Following Chimeric Antigen Receptor T-Cell Therapy in Multiple Myeloma”, by Fortuna et al. with great interest [1]. We found the article to be both comprehensive and highly valuable, especially in view of the current paucity of information on side effects of CAR T-cell therapies. However, we would like to offer additional observations that we believe are critical for a more accurate understanding of this condition. While the review acknowledges the risk of CAR T-cell lymphoma, we believe that indolent CAR T- cell lymphoproliferative disorders remain underrecognized and warrant greater emphasis due to their clinical significance.

While CAR T-cell therapies continue to advance, emerging data suggest that their long-term effects, including secondary malignancies, remain incompletely understood. To our best knowledge, four cases of CAR T-cell related indolent intestinal T cell lymphomas have been reported, though we suspect many additional undiagnosed cases exist due to overlapping clinical and histological features with autoimmune enteritis (AE) [2–5]. It is a rare and novel entity with histological nuances that make differentiation from AE particularly challenging.

We have recently published detailed clinical and pathologic features of an indolent intestinal T cell lymphoma case who presented with non-bloody diarrhea after cilta-cel therapy, a case also mentioned in the above article [2]. The patient’s biopsies were first evaluated by experienced pathologists and were presumed as AE. Given the AE diagnosis, glucocorticoid treatment was initiated. While transient improvement occurred with steroid administration, significant clinical deterioration led to further investigations. The histology of the original and follow-up biopsies was characterized by villous atrophy with small lymphocytic proliferation with minimal cytologic atypia in the lamina propria, occasional eosinophils, occasional intraepithelial lymphocytes and apoptotic bodies within the epithelium, and even focal neutrophilic cryptitis, which was still suggestive of AE (Fig. 1) [2]. However, one distinguishing feature was the accumulation of CD4+T cells exhibiting a granzyme-positive phenotype, which strongly favored CAR T-cell origin and further workup, including molecular studies later proved this to be CAR T-cell lymphoma.

Fig. 1. Detection of CAR T-cells using anti-camelid VHH antibody by immunohistochemistry.

A, B Duodenal biopsy from one of the follow-up biopsies of our patient, showing persistent CAR T-cell infiltration. C, D Colonic biopsy demonstrating ongoing CAR T-cell persistence despite benign appearance. Hematoxylin and eosin (H&E) staining is shown on the left, and corresponding anti-camelid VHH antibody staining is shown on the right (50× and 200×). Black arrows indicate apoptotic epithelial cells, blue arrows indicate intraepithelial lymphocytes, further were proven as CAR T+ lymphocytes with anti-camelid staining on the right.

Another patient from the Memorial Sloan Kettering Cancer Center, as presented by Perica et al., manifested diarrhea after cilta-cel therapy, histologically resembling AE, and was initially described as immune-mediated enterocolitis [3]. Of note, histology of this case also exhibited granzyme+ cytototoxic CD4+T cell phenotype. The patient was unresponsive to steroids and infliximab but was reported to have significant improvement with ruxolitinib therapy. Unfortunately, due to infectious complications, the patient passed away. Further molecular studies confirmed CAR T-cell malignancy. A similar enterotropic CAR+T-cell lymphoma case from Stanford University was reported by Hosoya et al. at the annual American Society of Hematology Meeting—San Diego, 2024 [4]. This patient also presented with diarrhea and exhibited comparable pathological features resembling AE. The patient was diagnosed with T cell lymphoma after a second EGD due to unresponsiveness to steroids, infliximab, and ustekinumab. Additionally, a study from Germany by Braun et al. documented a patient who developed both cutaneous and intestinal T-cell lymphoma [5]. Since the dermatologic manifestations, which were dominantly plaque formation, were pointing to T cell lymphoma, intestinal manifestations were investigated considering T cell malignancy. Despite extensive investigation and treatment, the patient succumbed to complications. These cases underscore the urgent need for increased awareness and research into resistant diarrhea cases with histopathologic features of autoimmune/immune-mediated enterocolitis.

Our understanding, based on these cases and histology of our patient’s follow-up biopsies, suggests that indolent intestinal CAR T-cell lymphoma has histological similarities to AE but appears distinct due to absence of plasma cells and B cells and accumulation of activated granzyme-positive cytotoxic CD4 and/or CD8+T cells. Unlike other intestinal T-cell lymphomas, it does not exhibit atypia in the neoplastic lymphocytes and its morphology may even seem deceptively normal at first glance, as emphasized by us and Perica et al. [2, 3]. Mass formation and tissue destruction may or may not be present on initial biopsies due to inadequate sampling or partial response to previous treatments. We propose that indolent intestinal CAR T-cell lymphoproliferative disorder/lymphoma constitutes a distinct clinicopathological entity, and it must be defined and diagnosed promptly to select the best therapy. We believe that cases with resistant diarrhea warrant additional workups, including prompt immunohistochemical analysis for the presence of CAR T-cells using commercially available anti-camelid VHH antibody (as shown in Figure 2 of the current article) [1]. Using the described protocol, we demonstrated the presence of CAR T-cells not only in the affected duodenum but also in histologically normal-appearing colon and confirmed clonal identity in follow-up biopsies from our patient (Fig. 1). Clonality can also be assessed using PCR-based T-cell receptor gene rearrangement assays on DNA extracted from small biopsy specimens. Additionally, integration site analysis is feasible using whole-genome sequencing, even from limited tissue. If clonality determination by PCR is not feasible promptly, immunohistochemistry with anti-camelid VHH antibody may show the presence of CAR T-cells in the tissue quickly, and we believe these cases should be diagnosed as indolent CAR T-cell lymphoproliferative disorder or CAR T-cell lymphoma if clonality is proven.

As for clonality, we also want to point out a possible diagnostic pitfall depicted in Figure 2 in the paper [1]. Here, the assumption is that lymphoma can be ruled out based on the presence of both CD8+ and CD8- T cells. This may lead to diagnostic oversight. Hosoya et al. documented a bi-clonal lymphoma with one major CD8+ and minor CD4+ clone. Braun et al. documented three different clones in their patient [4, 5]. These cases demonstrate that this argument is not absolute. Given the serious implications of diagnosing CAR T-cell lymphoma/lymphoproliferative disorder as immune-mediated/autoimmune enterocolitis, and the delays in appropriate management and the risk of secondary infections such as CMV reactivation (reported in two CAR T-cell lymphoma cases following steroid and infliximab therapy), we strongly advocate for a more cautious approach in the assessment of such cases.

Additionally, we would like to further discuss one of the proposed mechanisms for the gastrointestinal involvement of BCMA CAR T-cells. The article suggests that this phenomenon may be due to BCMA expression on plasma cells, which are densely populated in enteric tissue. However, in our patient, plasma cells and B cells were notably depleted, and they were absent in the follow-up biopsies. Despite their absence, CAR T-cells still accumulated in the small intestine. Similar findings have been observed in other reported cases of BCMA CAR T-cell–associated lymphomas. This discrepancy raises important questions that remain unresolved. There may be alternative hypotheses not addressed in this review. One possibility is the presence of circulating autoimmune intestinal disease-related T-cell clones [6, 7]. These clones can exist in individuals—akin to monoclonal B-cell lymphocytosis—with or without intestinal inflammation or symptoms and may have predated CAR T-cell therapy, proliferating extensively due to CAR integration or indirect activation. Another hypothesis is the presence of a cross-reactive antigen on intestinal epithelial cells that interacts with the BCMA CAR structure, since we observed intestinal epithelial apoptosis and intraepithelial CAR T-cells in our patient biopsies similar to GVHD. Another possibility is secondary apoptosis of enterocytes by nearby activated T lymphocytes [8, 9]. While multiple hypotheses can be explored, we strongly believe that addressing this issue is crucial for developing targeted therapeutic strategies.

Looking at the data provided, the severity of the condition is evident; almost half of the patients died because of enterocolitis, and the clinical benefit from infliximab or vedolizumab is not higher than 50% of the patients. While the data presented is valuable, the small number of reported cases makes it difficult to draw firm conclusions about treatment efficacy, and the variable, some unsustained responses suggests a risk of recurrence. It should also be considered that all these treatments come with side effects risks which can cause morbidity and mortality. Perica et al. claimed the effectiveness of ruxolitinib, but long-term follow-up data is lacking since the patient died of sepsis two weeks later. Notably, both Stanford’s patient and our patient have benefited from cyclosporine, with clinical improvement. If a diagnosis of CAR T-cell lymphoma is established, cyclosporine may represent an alternative therapeutic option in selected patients.

To refine diagnostic accuracy and improve treatment strategies, we believe it is very important to re-evaluate the pathology specimens on all these reported cases. Immunohistochemistry with anti-camelid VHH antibody can be performed on a single slide, even when tissue samples are frozen and limited. Additionally, adopting appropriate terminology for this disease is essential. The term ‘immune effector cell-associated enterocolitis’ may be suitable in some contexts but should only be applied if CAR T-cell lymphoproliferative disorder has been ruled out. If VHH staining is positive on many cells, and clonality is demonstrated by PCR, then using the term ‘lymphoma’ becomes more appropriate to avoid misdiagnosis and to ensure that optimal treatment strategies are considered. It is also important to note that the absence of anti-camelid VHH antibody staining does not definitively exclude CAR T-cell–related lymphoma, as vector insertion may drive oncogenesis even without detectable CAR expression via other mechanisms [10]. In such a clinically suspicious case, a PCR assay may show clonality and support the lymphoma diagnosis, and further molecular workup would be necessary to determine the etiology.

In summary, we believe that some cases diagnosed as immune effector cell-associated enterocolitis may represent CAR T-cell lymphoproliferative disorders. We aim to raise awareness of this possibility, as defining diagnostic criteria and using appropriate terminology are essential for guiding therapy and supporting informed clinical decision-making.

Author contributions

KS: contributed to validation of immunostains and wrote the initial draft; HA: contributed to validation of immunostains; JJC: performed immunostains; TML: primary gastroenterologist for the patient. MO: organized the study, validated the immunostains. All authors edited and approved the final version.

Competing interests

The authors declare no competing interests.