Chimeric antigen receptor T-cell (CAR-T) therapy represents one of the most remarkable advances in biotechnology applied to oncology in recent years. Its initial success was evidenced in hematologic malignancies such as leukemias and lymphomas, where it demonstrated significant response rates. However, translating this strategy to the treatment of solid tumors still faces major obstacles, including limited efficacy, potential toxicity, and tumor recurrence.
One of the critical points in this challenge lies in selecting the appropriate tumor target. Mesothelin (MSLN) stands out as one of the most promising molecules in this context. It is a glycosylphosphatidylinositol-anchored membrane glycoprotein highly expressed in various solid tumors, including mesothelioma, ovarian, pancreatic, gastric, and lung cancers, but only at low levels in normal tissues such as pleura, pericardium, and peritoneum. This feature makes mesothelin an attractive target, although not without risks, since residual expression in healthy tissues may predispose to on-target/off-tumor toxicity.
The key to overcoming this limitation lies in fine-tuning CAR-T receptor affinity. A receptor with excessively high affinity may recognize even normal cells with low antigen expression, leading to toxicity. Conversely, receptors with very low affinity may fail to generate an effective antitumor response. Thus, affinity tuning becomes essential to achieve a potent, durable, and safe response against MSLN-positive solid tumors.
Affinity tuning: concept and relevance
The concept of affinity tuning is based on balancing binding strength and selectivity. Modulating the antigen-binding domain of CAR-T receptors, often represented by the single-chain variable fragment (scFv), determines the intensity with which T cells will recognize their target.
When affinity is too high, there is a risk of uncontrolled activation and recognition of normal cells with minimal mesothelin expression, causing severe toxicity. When affinity is too low, the T cell fails to properly recognize the tumor and cannot exert cytotoxic activity. The optimal affinity-tuned point is therefore one in which the CAR-T cell is sensitive enough to detect tumor cells with intermediate MSLN density but selective enough not to react to normal cells.
This adjustment is particularly relevant for solid tumors, where antigenic heterogeneity is a major escape mechanism. Balancing sensitivity and selectivity can be decisive in ensuring an effective and durable immune response.
Development of CAR-T cells targeting mesothelin
The development of mesothelin-targeted CAR-T cells has undergone several optimization phases. Starting from the parental antibody M912, affinity maturation techniques using phage libraries generated four human anti-MSLN variants with different affinity levels: LP12, HP4-11, HP4-41/LP6, and HP4-44/LP2.
These receptors were incorporated into lentiviral vectors and transduced into human CD3+ T cells. The result was the generation of third-generation CAR-T cells capable of optimized MSLN recognition. In vitro tests showed strong cytotoxic activity, accompanied by cytokine secretion (IFN-γ, IL-2, TNF-α) and clonal proliferation upon activation.
Among the variants, LP12 stood out, offering the best balance of affinity, antitumor efficacy, and safety. In murine models, a single administration of LP12 CAR-T cells was sufficient to eradicate MSLN-positive solid tumors, promote long-term CAR-T persistence, prevent tumor recurrence, and avoid off-target toxicities. These findings reinforce the idea that affinity tuning is central to expanding CAR-T applicability against solid tumors.
Mechanisms of persistent antitumor immunity
The antitumor response mediated by affinity-tuned CAR-T cells depends on interconnected mechanisms that sustain potency and persistence:
1. Specific recognition and efficient activation – Affinity tuning allows selective recognition of mesothelin on tumor cells, triggering pro-inflammatory cytokine release and direct tumor lysis.
2. Clonal proliferation – Once activated, CAR-T cells proliferate extensively, increasing the effector pool and immune pressure on the tumor.
3. Tumor infiltration – These cells migrate through circulation, infiltrate the tumor microenvironment, and exert cytotoxic activity locally, overcoming physical and immune barriers.
4. Differentiation into memory cells – A key feature is the ability to differentiate into effector memory T cells, maintaining long-term immune surveillance and readiness in case of relapse.
5. Clinical safety – Thanks to affinity adjustment, nonspecific toxicity is reduced, as CAR-T cells do not react to low levels of mesothelin in healthy tissues.
Together, these mechanisms explain how affinity-tuned CAR-T cells targeting MSLN can induce antitumor immunity that is potent, durable, and safe.
Challenges and complementary strategies
Although results with affinity-tuned anti-MSLN CAR-T cells are promising, challenges remain for their clinical consolidation.
One is the heterogeneity of MSLN expression across and within tumors. In cases of very low expression, responses may be limited. In such scenarios, combining affinity tuning with strategies such as cytokine armoring (IL-15, IL-7) or resistance to soluble mesothelin may enhance performance.
Another promising approach is designing receptors with fast binding kinetics, enabling effective recognition without prolonged activation. This “fly-kiss” model suggests that transient, moderate-affinity interactions are more advantageous than stable, high-affinity ones, which may cause T-cell exhaustion.
Finally, combining CAR-T therapy with other modalities, such as immune checkpoint inhibitors, may further enhance efficacy by modulating the tumor microenvironment and supporting persistent CAR-T activity.
A promising strategy against solid tumors
Affinity-tuned CAR-T therapy targeting mesothelin represents one of the most promising strategies to overcome the hurdles of treating solid tumors. The proper balance between sensitivity and selectivity maximizes cytotoxic efficacy, supports memory formation, and minimizes toxicity.
The development of LP12 CAR-T cells demonstrated the feasibility of achieving tumor eradication, long-term persistence, and relapse prevention in preclinical models without significant adverse effects. These advances reinforce that scFv affinity tuning is a fundamental path to expand the clinical reach of cellular immunotherapy, offering concrete prospects for future application in patients with MSLN-positive solid tumors.
Read also on our blog: CAR-T cell therapy for juvenile-onset autoimmune diseases: a promising future?
