Dengue Literature Review

Platelet-Associated Immunoglobulin ELISA

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Platelet-Associated Immunoglobulin ELISA

Overview

The platelet-associated immunoglobulin ELISA (PAIg ELISA) is a competitive solid-phase assay used to quantify the amount of IgG or IgM bound to the surface of circulating platelets. It is the primary method used to investigate immune complex-mediated thrombocytopenia in dengue haemorrhagic fever. Results are reported as ng of IgG or IgM per 10⁷ platelets, allowing normalization across different platelet concentrations.

In dengue research, elevated PAIgG and PAIgM reflect immune complexes of dengue virus antigen with anti-dengue IgG or IgM, deposited on platelets via direct dengue-platelet binding. This is distinct from autoimmune ITP (idiopathic thrombocytopenic purpura), where PAIgG reflects anti-platelet autoantibodies.

Assay principle (competitive format): Wells are coated with standard human IgG (or IgM). A platelet sample at defined concentration (5–20 × 10⁴ cells/μl) plus HRP-conjugated anti-human IgG Fc antibody (or anti-IgM Fc antibody) are added simultaneously. Platelet-associated IgG (or IgM) competes with the coated standard for the detector antibody. Higher PAIg reduces signal relative to calibration curve; values are read at OD 492 nm.

Key Points from Literature

PAIgG in secondary dengue infection

Oishi2003 - PAIgG and Thrombocytopenia in Secondary Dengue established that PAIgG is significantly elevated in secondary dengue and inversely correlates with platelet count (r = −0.570, P < 0.0001; n = 53). Key findings:

  • Anti-dengue virus IgG — not anti-platelet autoantibody — is responsible for PAIgG elevation: platelet eluates contain anti-dengue IgG specificity; circulating anti-platelet autoantibody was found in only 1/53 patients (1.9%)
  • PAIgG fell significantly from acute to convalescent phase, mirroring platelet count recovery
  • PAIgG level does NOT correlate with plasma HI titre (r = −0.130, P = 0.361) — indicating that circulating antibody concentration alone is insufficient; the rate-limiting factor is dengue antigen availability on platelet surfaces

PAIgM in secondary dengue infection — extension of the PAIgG finding

Saito2004 - PAIgG and PAIgM in Secondary Dengue extended the method to PAIgM in a larger prospective cohort (n = 78 secondary-infection patients):

  • PAIgM is also significantly elevated in secondary dengue and inversely correlates with platelet count (r = −0.231, P = 0.046)
  • Anti-dengue IgM activity confirmed in platelet eluates (OD 0.35 ± 0.20 vs. healthy controls OD 0.09 ± 0.05)
  • PAIgM (but not PAIgG) independently predicts DHF by logistic regression (P < 0.01); cut-off >20 ng/10⁷ platelets: sensitivity 48.6%, specificity 92.1%
  • PAIgG is the quantitatively dominant pathway: 19.2% of patients had normal PAIgG vs. 50.7% with normal PAIgM, confirming PAIgM plays a smaller but distinct role

Mechanistic interpretation

The PAIg ELISA data support a specific model of thrombocytopenia in secondary dengue:

  1. Dengue virus binds directly to circulating platelets (RT-PCR detects DENV RNA in 42.8% of platelet samples in Saito2004)
  2. Anti-dengue IgG and IgM in secondary infection deposit on dengue-coated platelets as immune complexes (PAIgG and PAIgM)
  3. PAIgG-coated platelets are cleared by macrophages via both Fc receptor- and complement receptor-mediated phagocytosis, and by complement-mediated platelet lysis
  4. PAIgM-coated platelets are cleared via complement receptor and complement-mediated lysis only — IgM pentamer structure precludes Fc receptor engagement; this pathway is FcγR-independent

Contradictions & Debates

  • The contribution of PAIgM relative to PAIgG to overall thrombocytopenia is debated: Saito2004 shows PAIgG is the dominant contributor (wider abnormality rate, stronger inverse correlation), but PAIgM shows stronger association with DHF severity by logistic regression.
  • Neither PAIgG nor PAIgM correlate with plasma HI titre, confirming that total antibody level is not the driver — but the precise trigger for immune complex deposition on platelets (viral RNA/antigen density on platelets, complement activation threshold) has not been characterised.

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