How is HLA (Human Leukocyte Antigen) DR used in diagnosing immune paralysis?

HLA-DR Measurement for Diagnosing Immune Paralysis

Monocyte HLA-DR (mHLA-DR) expression measured by flow cytometry serves as the primary diagnostic tool for immune paralysis in critically ill patients, with values ≤5,000 molecules/cell indicating clinically significant immunosuppression and ≤2,000 molecules/cell marking severe immune dysfunction. 1, 2

Pathophysiology and Rationale

After the initial inflammatory response in sepsis or critical illness, professional antigen-presenting cells (dendritic cells and macrophages) reduce expression of the MHC class II molecule HLA-DR, which is a hallmark of the immunocompromised state that develops 3. This decreased HLA-DR expression reflects a broader pattern of immune dysfunction characterized by:

• Expansion of immunosuppressive cell populations (immature PMNs, MDSCs, M2 macrophages) 3
• Production of anti-inflammatory cytokines (IL-10, TGF-β) 3
• T cell exhaustion and anergy 3
• Increased susceptibility to secondary infections 3

HLA-DR controls the interplay between innate and adaptive immunity and serves as a "global" biomarker of injury-associated immunosuppression 1. Importantly, routine inflammatory markers like C-reactive protein, procalcitonin, or leukocyte counts cannot adequately assess functional immunity 1.

Measurement Methods

Flow Cytometry (Standard Approach)

• Quantitative measurement of HLA-DR molecules per monocyte cell using standardized flow cytometry 1
• Expressed as molecules/cell or as percentage of positive cells
• Requires specialized laboratory equipment and trained personnel 2

Point-of-Care Testing (Emerging)

• Novel point-of-care flow cytometers with single-use cartridges now enable bedside measurement 2
• Provides rapid results for clinical decision-making
• Overcomes technical boundaries of traditional flow cytometry 2

Alternative: Gene Expression

• Quantitative PCR measurement of HLA-DRA mRNA expression correlates with protein levels and TNFα response 4
• HLA-DRA expression decreased by ~70% in septic patients compared to controls 4
• Positive correlation between HLA-DRA and TNFα response (r +0.67) 4

Diagnostic Thresholds and Clinical Interpretation

Three clinically relevant thresholds stratify immunosuppression severity 2:

Threshold 1: ≤8,000 molecules/cell

• Indicates mild immunosuppression
• Found in 85.5% of ICU patients on admission 2
• Less specific for predicting complicated courses

Threshold 2: ≤5,000 molecules/cell

• Indicates moderate immunosuppression with clinical significance 2
• Present in 68.2% of ICU admissions 2
• Discriminates patients with:
  • Longer ICU stay 2
  • Extended ventilation time 2
  • Prolonged antibiotic therapy duration 2
  • Higher microbiological findings 2

Threshold 3: ≤2,000 molecules/cell

• Indicates severe immunosuppression 2
• Found in 40% of ICU patients 2
• Associated with:
  • Significantly longer ICU stay 2
  • Extended mechanical ventilation 2
  • Higher incidence of antibiotic therapy 2
  • Increased secondary infection risk 1

Clinical Application Algorithm

Step 1: Measure mHLA-DR on ICU admission

• Use flow cytometry (standard or point-of-care) 2
• Single measurement is sufficient for initial risk stratification 2

Step 2: Stratify risk based on threshold

• 5,000 molecules/cell: Low risk, standard monitoring

• 2,000-5,000 molecules/cell: Moderate risk, enhanced surveillance for infections 2
• <2,000 molecules/cell: High risk, consider for immunomodulatory interventions 1, 2

Step 3: Correlate with functional assays (if available)

• Ex vivo LPS-stimulated TNFα release provides functional confirmation 4
• TNFα:HLA-DRA ratio correlates negatively with SOFA score 4

Step 4: Monitor for clinical consequences

• Secondary/nosocomial infections 3
• Viral reactivation 3
• Prolonged ICU course 2
• Development of persistent inflammation, immunosuppression, and catabolism syndrome (PICS) 3

Important Caveats

HLA-DR measurement does not predict mortality 2. While decreased HLA-DR is associated with adverse outcomes including secondary infection risk, neither the 2,000 nor 5,000 molecules/cell threshold stratified patients by mortality in validation studies 2.

Immunosuppression is independent of admission diagnosis 2. Large proportions of ICU patients present with decreased HLA-DR regardless of whether they have sepsis, trauma, or post-surgical inflammation 2.

The threshold for defining "immunoparalysis" remains debated 3. While <5,000 molecules/cell is commonly used, some experts advocate for lower thresholds, and the optimal cutoff may vary by clinical context 1, 2.

Injury-associated immunosuppression is potentially reversible, opening therapeutic avenues for affected patients 1. This makes accurate diagnosis particularly important for identifying candidates for immunomodulatory interventions.