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The systematic review by Mahmoudi and colleagues arrives at a moment when the field of CAR T-cell therapy is confronting a fundamental question about its manufacturing paradigm. Conventional CAR T-cell therapy, exemplified by approved products for B-cell malignancies and multiple myeloma, requires harvesting a patient’s T cells through leukapheresis, shipping them to a centralized manufacturing facility, genetically engineering them over several weeks, performing quality control testing, and then infusing the final product back into the patient. This process is expensive, logistically complex, and subject to failures related to the quality of the patient’s own cells, which are often compromised by prior chemotherapy, lymphodepletion, and the biology of the underlying disease itself. In vivo CAR T-cell engineering proposes to collapse this entire workflow into a single injection that reprograms T cells inside the patient’s body, potentially transforming a bespoke cell therapy into something closer to an off-the-shelf biologic.

The distinction between the two delivery platforms identified in the review—lipid nanoparticles carrying CAR-encoding messenger RNA and integrating lentiviral vectors—reflects two fundamentally different philosophies about how in vivo programming should work. Messenger RNA delivered by lipid nanoparticles produces a transient expression of the CAR on the surface of T cells, because the mRNA does not integrate into the genome and is gradually degraded by cellular machinery. This transience is a double-edged sword: it may limit the duration of the therapeutic effect and require repeat dosing, but it also provides an inherent safety mechanism, since any toxicity related to CAR expression should diminish as the mRNA is cleared. Lentiviral vectors, by contrast, integrate the CAR construct into the genome of the transduced cells, producing stable, potentially lifelong expression. This durability is attractive for malignancy treatment, where persistent anti-tumor activity is desired, but it raises the specter of uncontrolled in vivo expansion, since the vector cannot be restricted to a specific cell population with the precision achievable in an ex vivo manufacturing setting.

The targeting strategies used to direct these agents to T cells deserve particular attention. The review’s abbreviation list reveals the use of a CD8-targeted lipid nanoparticle, which exploits antibodies or ligands that bind the CD8 molecule expressed on cytotoxic T lymphocytes. This approach attempts to concentrate the payload on the cell type most likely to mediate tumor killing, while sparing CD4-positive helper cells, B cells, and other immune populations. The in vivo Gene Placement System referenced in the abbreviations suggests an alternative strategy in which the vector itself is engineered to preferentially integrate or express in T cells, perhaps by exploiting T-cell-specific promoters or miRNA-regulated expression cassettes that silence the transgene in non-T-cell lineages. These targeting mechanisms are critical because off-target transduction of non-T cells could produce unintended consequences, including expression of the CAR on antigen-presenting cells, which could theoretically trigger immune dysregulation or fratricidal effects.

The clinical results summarized in the review, while preliminary, offer encouraging signals across both malignant and autoimmune indications. In relapsed or refractory multiple myeloma, the achievement of minimal residual disease negativity is particularly noteworthy, as MRD negativity is one of the most powerful predictors of long-term survival in myeloma and is generally only achieved with deep, sustained anti-myeloma activity. The fact that in vivo-generated CAR T cells could produce this depth of response suggests that the endogenous T cells, once reprogrammed, are capable of mounting effective anti-tumor responses comparable in some respects to those achieved with ex vivo-manufactured products. In B-cell non-Hodgkin lymphoma, objective responses were observed, which is consistent with the known biology of CD19-directed CAR T-cell therapy in this disease, although the specific target antigen used in the in vivo trials is not detailed in the abstract.

The inclusion of autoimmune diseases in the review reflects a broader trend in the CAR T-cell field, where the deep B-cell depletion achieved by CD19-directed CAR T cells has been repurposed to treat conditions such as systemic lupus erythematosus, in which autoreactive B cells drive pathology. The observation of improved disease activity across autoimmune cohorts in this review aligns with case reports and small series published by other groups, suggesting that in vivo CAR T-cell generation may eventually offer a safer, more accessible alternative to ex vivo products for autoimmune indications, where the risk-benefit calculus differs from that in advanced malignancy.

The safety profile reported in the review warrants careful interpretation. Cytokine release syndrome occurred in 24 of 31 patients in studies reporting numerical incidence, which is a high proportion, although the review notes that most events were low grade. This is consistent with the known pharmacology of CAR T-cell therapy, in which the activation of engineered T cells triggers a cascade of inflammatory cytokines including interleukin-6, interferon-gamma, and tumor necrosis factor-alpha. The occurrence of severe events, even if uncommon, underscores that in vivo CAR T-cell generation does not eliminate the fundamental risk of on-target inflammatory toxicity. The rarity of neurotoxicity, with only one grade 1 event reported across the included studies, is notable and may reflect several factors, including the small sample size, the transient nature of mRNA-based CAR expression, or differences in the cytokine milieu generated by in vivo versus ex vivo engineered cells. However, given the small numbers of patients involved, it would be premature to conclude that in vivo CAR T-cell therapy is inherently less neurotoxic than conventional approaches.

The methodological limitations acknowledged by the authors are substantial and deserve emphasis. Seven clinical trials involving 41 patients represent an extremely small evidence base, and the heterogeneity of the included studies—in terms of disease types, delivery platforms, target antigens, doses, and patient populations—makes it impossible to draw meaningful comparative conclusions. The short follow-up reported is particularly concerning for malignancy indications, where durability of response is the key clinical question. A patient with relapsed myeloma who achieves an initial complete response but relapses within three months has not benefited meaningfully from the therapy, and the review provides insufficient information to assess whether the responses observed are durable. The authors’ call for standardized reporting of cellular kinetics is well taken, as the expansion and persistence characteristics of in vivo-generated CAR T cells are likely to differ substantially from those of ex vivo products, and understanding these differences is essential for optimizing dosing schedules and predicting toxicity.

Looking forward, several challenges must be addressed before in vivo CAR T-cell therapy can advance beyond early-phase testing. First, the efficiency and specificity of T-cell targeting must be demonstrated rigorously in larger cohorts, with careful biodistribution studies to quantify off-target transduction. Second, the pharmacokinetics and pharmacodynamics of repeat dosing, particularly for mRNA-based platforms, need to be characterized, including whether repeated administration leads to immune responses against the lipid nanoparticle or the mRNA itself. Third, the question of dose control is paramount: in conventional CAR T-cell therapy, the dose is precisely known because the cells are counted before infusion, whereas in vivo generation means that the actual number of CAR-positive T cells produced depends on transduction efficiency, T-cell fitness, and host factors that vary from patient to patient. Fourth, the regulatory pathway for in vivo CAR T-cell products remains uncertain, as these agents blur the traditional distinction between gene therapy, cell therapy, and drug products, potentially requiring novel regulatory frameworks. Finally, the cost-effectiveness of in vivo approaches, while potentially superior to ex vivo manufacturing in the long run, has not been established, and the infrastructure required for quality control and batch release of in vivo products may prove more complex than anticipated.

The systematic review by Mahmoudi and colleagues thus serves as an important early map of a rapidly evolving landscape. It documents that in vivo CAR T-cell generation is biologically feasible and capable of producing clinically meaningful responses, while honestly acknowledging the limitations of the current evidence base. As more trials mature and additional platforms enter clinical testing, the field will need to move beyond proof-of-concept toward rigorous comparative studies that can determine whether in vivo CAR T-cell therapy can match the efficacy of established ex vivo products while delivering on its promise of broader accessibility, reduced cost, and improved safety.

Subject of Research: …

Article Title: In vivo CAR T-cell therapy: a systematic review of clinical trials

Article References: Mahmoudi, E., Shokrolahi, F., Khoshkanab, A. H. R., & Namjoo, S. (2026). In vivo CAR T-cell therapy: a systematic review of clinical trials. BMC Cancer. https://doi.org/10.1186/s12885-026-17165-x

Image Credits: AI Generated

DOI: 10.1186/s12885-026-17165-x

Keywords: vivo, T-cell, therapy, systematic, review, clinical, trials, scientific research