CD19-targeted therapies now reach patients with relapsed or refractory large B-cell lymphoma (LBCL) who can’t receive CAR T-cell therapy or who progress after it. Because these agents share one target, the next treatment may meet a tumor already shaped by the last, and it is unclear whether prior CD19 exposure blunts a second CD19-directed therapy.
A recent review of CAR T-cell therapy, tafasitamab plus lenalidomide, and loncastuximab tesirine covers clinical trials, real-world series, and emerging data on CD19 loss. Each drug engages CD19 differently: CAR T cells need antigen recognition to trigger T-cell killing, tafasitamab recruits natural killer cells and macrophages, and loncastuximab delivers a cytotoxic payload once internalized.
Antigen density and timing between treatments may matter differently for each agent, most of all in heavily pretreated or high-risk disease. Prior CD19-targeted therapy does not automatically rule out another, yet preserved antigen expression and differing binding epitopes do not guarantee a response. The data are small retrospective cohorts, and no trial compares the agents directly.
CD19-Targeted Therapies
CD19 is expressed throughout much of normal B-cell development and in most B-cell malignancies, and it is absent from hematopoietic stem cells and most non-B-cell tissues, which has made it an established target in lymphoma.
CD19-directed CAR T-cell therapies, including axicabtagene ciloleucel (axi-cel), lisocabtagene maraleucel (liso-cel), and tisagenlecleucel (tisa-cel), are active in relapsed or refractory diffuse large B-cell lymphoma, and axi-cel and liso-cel are established second-line options for primary refractory disease or relapse within 12 months of first-line chemoimmunotherapy. Access is limited by manufacturing requirements, referral delays, cost, and toxicities. Estimates cited in the review suggest that only 10%-30% of eligible patients in the United States, European Union, and United Kingdom receive CAR T-cell therapy.
Tafasitamab with lenalidomide and loncastuximab tesirine can both be given in outpatient settings and are active in patients who are ineligible for CAR T-cell therapy or need further treatment. In the phase 2 L-MIND trial, the combination achieved an overall response rate of 58%, including 41% complete responses. Median progression-free survival was 11.6 months and median overall survival was 33.5 months, with better outcomes after only one previous line of therapy. In a retrospective real-world population with more high-risk features and comorbidities, the ORR was 31%, median PFS 1.9 months, and median OS 6.5 months.
Loncastuximab tesirine is an antibody-drug conjugate with a pyrrolobenzodiazepine payload. In the phase 2 LOTIS-2 trial in heavily pretreated patients, it achieved an ORR of 48%, including 25% CR, with a median OS of 9.5 months. A real-world analysis reported an ORR of 32% and median OS of 4.6 months. Differences in populations, prior treatment, and eligibility prevent direct comparison between these agents and CAR T-cell therapy.
The Clinical Relevance of CD19 Expression
The relationship between antigen expression and response is more complex than a positive or negative result.
Loncastuximab may retain activity at low antigen density. Preclinical studies showed cytotoxicity against lymphoma cell lines with low or undetectable CD19 by immunohistochemistry, possibly through efficient internalization and a bystander effect from its diffusible payload. Quantitative pharmacology models predicted activity at roughly 1,000 CD19 molecules per cell, a preclinical estimate that has not been established as a clinical threshold.
Limits of CD19 Testing
The lack of standardized, sufficiently sensitive CD19 testing is a major obstacle. Immunohistochemistry is widely available but may not distinguish membrane-bound from intracellular CD19 and can miss low expression. In ZUMA-1, pretreatment CD19 by immunohistochemistry did not correlate directly with response to axi-cel, and in 59 patients from LOTIS-2 it did not predict response to loncastuximab. This may reflect inadequate assay sensitivity and not irrelevance of antigen density.
Flow cytometry is more sensitive and can quantify surface antigen. In 22 patients treated with axi-cel, median pretreatment CD19 was approximately 6,400 molecules per cell, and patients with 3,000 or fewer had a significantly higher risk of progression. Quantitative polymerase chain reaction is another option when fresh tissue is unavailable: in one comparison, it detected CD19 in six of 16 samples classified as CD19-negative by immunohistochemistry. It measures transcripts and not functional surface antigen.
No method has an accepted threshold for selecting or excluding patients. A negative immunohistochemistry result alone should not exclude a patient from further CD19-directed therapy.
How Resistance Develops
CD19-negative or CD19-low relapse is an established mechanism of resistance after CAR T, through genetic or transcriptional changes, epitope masking, and selection of tumor populations with reduced antigen. CD19 mutations and alternative splicing can change the extracellular structure and prevent recognition, but ZUMA-1 analyses found few relevant splicing events and most relapse biopsies retained full-length CD19.
Epitope masking matters mainly for tafasitamab, which binds the 4G7 epitope, partially overlapping the FMC63 epitope recognized by conventional CD19 CAR T cells. Preclinical studies show it can temporarily block CAR T-cell recognition, and CAR T-cell proliferation recovers as tafasitamab concentrations fall.
Selective pressure is the other mechanism: CAR T-cell therapy may eliminate CD19-high cells and leave CD19-low populations to expand. In ZUMA-1, 28% of post-relapse biopsies showed reduced or absent CD19, and across studies CD19 loss was reported in approximately 8%-30% of patients relapsing after CAR T.
Evidence of CD19 loss after tafasitamab is inconsistent, and limited data show no substantial loss after loncastuximab. These differences give a biological rationale for sequencing but do not show which sequence gives the best outcomes.
Bridging to CAR T-Cell Therapy
Patients awaiting CAR T-cell therapy may need treatment to control rapidly progressive lymphoma. Manufacturing takes about two to four weeks, but referral-to-infusion intervals are often longer. Another CD19-directed agent in this period raises concern about downregulation or masking, and available retrospective studies are somewhat reassuring.
After tafasitamab-lenalidomide, nine patients who received axi-cel or tisa-cel had an ORR of 78%, including 44% CR. In the French DESCAR-T registry, 15 patients who received tafasitamab-lenalidomide before CAR T-cell therapy had an ORR of 46%, including 36% CR, with median PFS of three months.
After loncastuximab, a real-world study of 16 patients reported a CAR T-cell ORR of 63%, including 44% CR, and another retrospective analysis of 14 patients found a 43% CR rate. In the latter, CD19 remained detectable by immunohistochemistry in all ten patients tested before infusion.
Prior exposure does not necessarily prevent CAR T-cell responses. The cohorts are small and differ in disease characteristics, so the evidence supports feasibility but does not make either agent a preferred bridge or exclude clinically meaningful effects on CAR T-cell outcomes.
After CAR T-Cell Failure
Treatment after CAR T-cell relapse is difficult, particularly in rapidly progressive or heavily pretreated disease. CD20×CD3 bispecific antibodies are important options. Results for tafasitamab-lenalidomide have been disappointing. In DESCAR-T, 52 patients treated after CAR T-cell therapy had an ORR of approximately 14%, with 8% CR, median PFS of three months, and median OS of 4.7 months.
Loncastuximab has shown activity, with widely varying outcomes. In a post hoc LOTIS-2 analysis, three of 14 patients previously given CAR T-cell therapy achieved CR. A real-world analysis of 112 patients treated after CAR T-cell failure reported an ORR of 30%, including 15% CR.
Another retrospective study reported an ORR of 73%, including 34% CR, in 95 patients given third-line loncastuximab after second-line CAR T-cell therapy, and an ORR of 78% with a 17% CR rate in 23 patients given fourth-line loncastuximab after third-line CAR T-cell therapy. A pooled analysis of loncastuximab and tafasitamab-based regimens after CAR T-cell therapy found an ORR of 27% and median PFS of approximately eight weeks. Further CD19-targeted therapy can remain effective after CAR T-cell failure, particularly loncastuximab in selected patients.
Choosing a Sequence
The authors base treatment selection on disease characteristics, prior therapy, expected toxicity, availability, and the patient’s condition. For eligible patients in the second-line setting, CAR T-cell therapy remains the established standard, and the availability of tafasitamab-lenalidomide or loncastuximab should not delay referral.
The authors favor repeat biopsy and sensitive CD19 assessment before further CD19-targeted treatment. Antigen expression alone can’t determine the best treatment. CAR T-cell efficacy also depends on T-cell fitness, expansion, and persistence, which may explain why the same antigen does not predict equivalent responses across therapies.
Future Clinical Considerations
Ongoing trials are testing CD19-directed therapies earlier in LBCL, including tafasitamab-lenalidomide with R-CHOP in the frontline setting and axi-cel in newly diagnosed high-risk disease. If these become established, more patients with relapsed disease will already have been exposed to CD19-directed therapy, and the significance of repeated exposure will grow.
Prospective sequencing studies are needed on the timing of CD19-targeted agents before CAR T-cell therapy, the relevance of tafasitamab-associated masking, and the comparative effectiveness of CD19-directed treatments after CAR T-cell failure. Dual-target CAR T cells, CAR-NK cells, and CD19-directed bispecific or trispecific engagers may add options, and their use will require a better understanding of cross-resistance and antigen escape.
Reference
Caimi PF, Danesi R, Iacoboni G, et al. Sequencing CD19-targeted therapies in patients with relapsed/refractory large B-cell lymphoma: A narrative review. Clinical Lymphoma, Myeloma & Leukemia. 2026.
