Fcγ Receptors I and III on Splenic Macrophages Mediate GPIIb/IIIa Autoantibody-Dependent Phagocytosis of Platelets in Human Immune Thrombocytopenia
Bibliographic record
Abstract
Abstract Autoantibody-opsonized platelets in immune thrombocytopenia (ITP) are thought to be destroyed primarily by macrophage Fc gamma receptor (FcγR)-mediated phagocytosis in the spleen. Blockade of splenic macrophage FcγRs has been proposed as a therapeutic mechanism for ITP intervention. Unfortunately, the contribution of specific FcγRs to disease in ITP remains unknown. Our objective was to determine which FcγRs are responsible for the phagocytosis of ITP autoantibody-opsonized platelets by splenic macrophages. Splenic macrophages were purified by CD14 positive selection from spleens of splenectomized ITP patients, and were treated with blocking antibodies to FcγRI, FcγRIIa, FcγRIIa/b/c, and FcγRIII. Blocking antibodies were deglycosylated to prevent non-specific blocking effects by their Fc region. Two separate ITP sera confirmed positive for anti-GPIIb/IIIa autoantibodies by the monoclonal antibody immobilization of platelet antigens (MAIPA) assay were used to opsonize healthy donor human platelets. Phagocytosis was determined by confocal microscopy and non-phagocytosed (external) platelets were differentiated by an anti-platelet antibody stain following macrophage fixation. Human ITP splenic macrophages were found to express FcγRI, FcγRIIa, FcγRIIa/b/c, and FcγRIII, and expression was not significantly different compared to healthy (trauma) controls (n=5). The two anti-GPIIb/IIIa-positive ITP sera induced a mean 3.7- and 4.2-fold increase of platelet uptake by ITP splenic macrophages relative to normal human serum controls (n=3 each, p<0.001). Blockade of all FcγRs significantly reduced phagocytosis of serum-opsonized platelets for both ITP sera as compared to an IgG control (p<0.001) down to background (non-opsonized) levels. Using single blocking antibodies, inhibition of FcγRI reduced splenic macrophage phagocytosis of platelets by 45% and 37% for the two respective ITP sera as compared to IgG controls (n=3 for each patient sera, p<0.01), while inhibition of FcγRIII reduced phagocytosis of platelets by 43% and 44% (n=3 each, p<0.05). Blockade of FcγRIIa or FcγRIIa/b/c only marginally inhibited splenic macrophage phagocytosis and was not significant. An Fab-like FcγRIII-blocking antibody with a mutant Fc region completely deficient for FcγR binding demonstrated equal FcγRIII blockade compared to the deglycosylated anti-FcγRIII antibody. In comparison to ITP sera-opsonized platelets, FcγRI had a greater involvement in ITP splenic macrophage phagocytosis of anti-D-opsonized human erythrocytes as FcγRI blockade inhibited phagocytosis by 70% (n=5, p<0.001), while FcγRIII blockade inhibited phagocytosis by only 30% (p<0.001) and FcγRIIa or FcγRIIa/b/c blockade had no significant effect. This work demonstrates that FcγRI and FcγRIII are the primary phagocytic receptors on splenic macrophages for anti-GPIIb/IIIa autoantibody-opsonized platelets in human ITP and suggests that FcγRI and FcγRIII are the best targets for FcγR blockade as a potential therapeutic maneuver in the treatment of ITP. Disclosures No relevant conflicts of interest to declare.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.002 | 0.001 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".