Passenger Lymphocyte Syndrome: Updated Insights Into an Overlooked Transplant Complication

Passenger lymphocyte syndrome (PLS) is becoming increasingly relevant as transplant centers expand the use of ABO-incompatible procedures to address persistent organ shortages. Although first described decades ago, PLS continues to challenge clinicians due to its unpredictable presentation and the absence of unified treatment standards. A recent review offers a clear synthesis of current knowledge, outlining how PLS develops, how it can be recognized early, and where the field is headed.

Understanding PLS: A Growing Concern in Transplant Medicine

Transplantation remains one of the most effective treatments for end-stage organ disease, and advances in ABO-incompatible transplantation have expanded the donor pool. These gains come with new complications—PLS being among the most notable.

PLS occurs when donor-derived memory B lymphocytes transferred with an organ or stem cell graft begin producing antibodies directed against the recipient’s red blood cells or platelets. This can lead to hemolytic anemia or thrombocytopenia.

The rise in PLS cases is linked to several factors:

• Broader use of ABO-incompatible transplants

• Better clinical monitoring

• Increased awareness of immune-mediated post-transplant complications

There is no confirmed age or sex predisposition. However, children may be more susceptible to severe hemolysis because of differences in immune system maturity. Other important risk factors include grafts rich in lymphoid tissue (such as liver and intestine), donors previously sensitized through transfusion or pregnancy, and insufficient immunosuppression that allows donor B cells to remain active.

How PLS Develops: Three Key Steps

The review describes PLS as a process unfolding in three stages:

1. Transfer of donor lymphocytes

Immunocompetent B cells from the graft migrate into the recipient’s bloodstream. Liver and intestinal grafts carry especially high lymphocyte loads. Lung grafts can transfer donor cells capable of producing anti-D antibodies.

2. Antibody production

These donor-derived B cells begin producing antibodies against the recipient’s red blood cell antigens.
Common targets include:

• Anti-A/B in ABO-incompatible transplants

• Anti-D in Rh incompatibility

• Platelet antigens in select cases

Donors who were previously alloimmunized—for example, those with antibodies such as anti-K or anti-Jk(a)—pose an increased risk.

3. Hemolysis or thrombocytopenia

The produced antibodies activate complement or trigger Fc-mediated phagocytosis, destroying red blood cells or platelets. The severity of cell destruction depends on antibody levels and antigen density on the recipient’s blood cells.

Recognizing PLS: Clinical Features and Diagnostic Approach

PLS usually appears within days to weeks after transplantation. Common signs include:

• Hemolytic anemia

• Elevated LDH

• Indirect hyperbilirubinemia

• Reduced haptoglobin

Diagnosis relies on clinical features combined with laboratory testing, especially the direct antiglobulin test (DAT) and antibody screening.

Because its symptoms overlap with other post-transplant conditions, distinguishing PLS from drug-induced hemolysis or transplant-associated thrombotic microangiopathy is essential. The review emphasizes detailed laboratory evaluation and careful clinical correlation, supported by a structured monitoring approach.

Treatment Strategies: Current Practice and Challenges

Management aims to halt hemolysis and stabilize blood counts.

First-line measures include:

• Donor-compatible red blood cell transfusions

• High-dose corticosteroids

For refractory cases, options may include:

• Rituximab

• Plasmapheresis

Most cases are self-limiting, but the absence of standardized protocols means treatment approaches vary widely between institutions. The authors highlight that overly aggressive early therapy may not be necessary in mild cases, underscoring the importance of balanced clinical judgment.

Looking Ahead: What the Field Needs Next

The review identifies several key areas for future progress:

• Better biomarkers
  Genomic and proteomic tools could help predict PLS risk and improve early diagnosis.

• Mechanism-driven therapies
  Targeted treatments against donor-derived lymphocytes could reduce reliance on broad immunosuppression.

• Consensus guidelines
  Unified diagnostic and treatment frameworks would improve patient outcomes and reduce variability across centers.

• Improved monitoring
  Collaboration among transplant immunologists, hematologists, and computational specialists may help shift PLS from a reactive challenge to a preventable one.

Why These Insights Matter

As transplantation continues to evolve, PLS represents a critical intersection of immunology and clinical practice. Understanding its mechanisms and improving diagnostic accuracy can help protect graft function and patient well-being. This review underscores the need for continued research, clearer clinical pathways, and more precise tools—setting the stage for more consistent and effective management of this under-recognized complication.