Claim

In autoimmune inflammatory disease receiving rituximab induction, a model that combines early CD4 nadir (<300/uL) and absolute lymphocyte decline will predict Pneumocystis jirovecii pneumonia (PJP) more accurately than glucocorticoid dose-duration thresholds alone.

Why this matters

Current prophylaxis decisions often hinge on prednisone dose and duration, but recent rheumatology data suggest the highest-risk phenotype is rituximab plus prolonged high-dose glucocorticoids. That still leaves an unresolved question: among patients receiving similar steroid exposure, who is biologically most vulnerable? Cellular immune depletion may provide the missing discriminant.

Mechanistic rationale

• Rituximab does not directly deplete T cells, but in severe autoimmune disease it is often paired with steroid-intensive induction and profound inflammatory stress.
• PJP is classically linked to impaired cell-mediated immunity.
• Therefore, trajectory of lymphocyte/CD4 suppression may capture susceptibility more directly than steroid dose alone.

Testable prediction

In a prospective multicenter autoimmune cohort treated with rituximab:

1. A baseline model using only steroid dose and duration will have lower discrimination for 6-month PJP than a model adding week-2 and week-4 CD4/ALC values.
2. The strongest signal will appear in AAV and ILD-associated phenotypes.
3. Patients with prednisone >=30 mg/day for >=4 weeks but preserved CD4/ALC will have lower realized PJP incidence than those with comparable steroid exposure and early cellular nadir.

Proposed study

• Population: adults with AAV, SLE, RA, IIM, or SSc starting rituximab
• Follow-up: 6 months
• Primary endpoint: adjudicated proven/probable PJP
• Covariates: prednisone exposure, lymphocyte count, CD4 count, ILD, age, eGFR, TMP-SMX exposure
• Analysis: time-dependent Cox model and AUROC comparison between nested models

Falsifiability

This hypothesis fails if adding dynamic CD4/lymphocyte data does not improve discrimination, calibration, or net benefit over steroid thresholds alone.

Limitations

• PJP is rare, so multicenter enrollment is required.
• CD4 testing is not standard in every rheumatology workflow.
• Prophylaxis itself changes observed incidence and must be modeled as a time-varying exposure.

References

1. Fragoulis GE, et al. 2022 EULAR recommendations for screening and prophylaxis of chronic and opportunistic infections in adults with autoimmune inflammatory rheumatic diseases. Ann Rheum Dis. 2023;82:742-753. DOI: 10.1136/ard-2022-223335
2. Fragoulis GE, et al. Systematic literature review informing the 2022 EULAR recommendations. RMD Open. 2022;8:e002726. DOI: 10.1136/rmdopen-2022-002726
3. Park JW, et al. Risk-benefit analysis of primary prophylaxis against PJP in rheumatic diseases receiving rituximab. Arthritis Rheumatol. 2023;75:2036-2044. DOI: 10.1002/art.42541
4. Aqeel F, et al. PJP prophylaxis in patients with ANCA vasculitis on rituximab maintenance therapy. Glomerular Dis. 2024;4:152-158. DOI: 10.1159/000539993
5. Limper AH, et al. ATS statement on diagnosis and management of PJP. Am J Respir Crit Care Med. 2021;204:1232-1248. DOI: 10.1164/rccm.202109-1983ST