Immunopharmacology: Foundations and Cytokine Pharmacology

Correct Answer

B — Ravulizumab

Rationale

Ravulizumab is an engineered successor to eculizumab that contains four amino acid substitutions extending its plasma half-life approximately four-fold by increasing affinity for the neonatal Fc receptor, which recycles immunoglobulin G and extends its circulating life. This allows maintenance dosing every 8 weeks compared to eculizumab's every 2 weeks, with equivalent clinical efficacy and the same approved indications. Eculizumab requires intravenous infusions every 2 weeks. Pegcetacoplan targets complement component C3, not C5, and is administered subcutaneously twice weekly. Avacopan is an oral C5a receptor 1 antagonist, not a C5 inhibitor.

Correct Answer

A — Avacopan

Rationale

Avacopan is classified as an oral small-molecule antagonist of C5a receptor 1 (also called CD88). It is approved for anti-neutrophil cytoplasmic antibody-associated vasculitis as a corticosteroid-sparing agent. Eculizumab is an intravenous monoclonal antibody that binds and blocks C5 protein directly. Iptacopan is an oral small molecule but targets factor B of the alternative pathway, not the C5a receptor. Pegcetacoplan is a pegylated cyclic peptide that binds C3 and C3b, acting upstream of C5.

Correct Answer

D — Abatacept

Rationale

Abatacept is classified as a recombinant fusion protein of the extracellular domain of cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) fused to the Fc region of human immunoglobulin G1. This structural class distinguishes it from monoclonal antibodies. Daratumumab is a fully human anti-CD38 monoclonal antibody. Ravulizumab is a humanized anti-C5 monoclonal antibody. Belimumab is a fully human monoclonal antibody targeting B lymphocyte stimulator.

Correct Answer

C — Daratumumab

Rationale

Daratumumab is classified as a fully human immunoglobulin G1 monoclonal antibody targeting CD38, a transmembrane glycoprotein highly expressed on plasma cells and malignant myeloma cells. Rituximab targets CD20 on B cells and is a chimeric rather than fully human antibody. Bortezomib is a small-molecule proteasome inhibitor, not a monoclonal antibody. Abatacept is a CTLA-4-Ig fusion protein that targets the co-stimulatory ligands CD80 and CD86 on antigen-presenting cells.

Correct Answer

A — Pegcetacoplan

Rationale

Pegcetacoplan is classified as a pegylated cyclic peptide inhibitor that binds complement component C3 and C3b directly, blocking all three complement activation pathways upstream of C5. By acting at C3, it prevents both C3b-mediated opsonization (reducing extravascular hemolysis) and downstream membrane attack complex formation (reducing intravascular hemolysis). Eculizumab targets C5 specifically. Avacopan is a small-molecule antagonist of the C5a receptor 1. Iptacopan is an oral factor B inhibitor that selectively blocks the alternative pathway amplification loop.

Correct Answer

B — Iptacopan

Rationale

Iptacopan is an oral factor B inhibitor. Factor B is the serine protease that combines with C3b to form the alternative pathway C3 convertase (C3bBb); by inhibiting factor B, iptacopan selectively blocks the alternative pathway amplification loop while preserving the classical and lectin pathways. It is approved as oral monotherapy for paroxysmal nocturnal hemoglobinuria. Avacopan is an oral C5a receptor 1 antagonist approved for anti-neutrophil cytoplasmic antibody-associated vasculitis. Pegcetacoplan targets C3 and C3b upstream of all pathways. Ravulizumab is an intravenous anti-C5 monoclonal antibody.

Correct Answer

B — Ravulizumab

Rationale

Ravulizumab is an engineered successor to eculizumab that shares the same C5-blocking mechanism and approved indications but has an approximately four-fold longer plasma half-life, allowing maintenance dosing every 8 weeks compared to eculizumab's every 2 weeks. It is the appropriate switch for patients seeking a less burdensome infusion schedule without changing mechanism or indication coverage. Pegcetacoplan targets C3/C3b rather than C5, uses a different mechanism, and is administered by subcutaneous infusion twice weekly — not less frequently than eculizumab. Avacopan is an oral C5a receptor antagonist approved for anti-neutrophil cytoplasmic antibody-associated vasculitis, not paroxysmal nocturnal hemoglobinuria. Iptacopan is an oral factor B inhibitor approved for paroxysmal nocturnal hemoglobinuria but acts by a different mechanism (alternative pathway inhibition), not C5 blockade.

Correct Answer

D — Avacopan blocks the C5a receptor rather than C5 itself, so C5 cleavage still occurs and membrane attack complex formation remains intact

Rationale

Terminal complement — specifically the membrane attack complex formed from C5b through C9 — is the primary bactericidal defense against Neisseria meningitidis. Eculizumab and ravulizumab block C5 directly, preventing its cleavage and therefore preventing both C5a generation and membrane attack complex formation; this loss of terminal complement necessitates meningococcal vaccination. Avacopan acts downstream of C5 cleavage: it antagonizes the C5a receptor 1, blocking only the pro-inflammatory effects of C5a (neutrophil chemotaxis and activation), while C5b generation and membrane attack complex assembly proceed normally. Because the bactericidal membrane attack complex is preserved, avacopan does not impose the same meningococcal risk as agents that block C5 itself, and mandatory meningococcal vaccination is not required.

Correct Answer

C — CTLA-4-Ig fusion protein; blocks CD80 and CD86 on antigen-presenting cells, preventing the co-stimulatory signal required for full T-cell activation

Rationale

Abatacept is classified as a CTLA-4-Ig fusion protein — the extracellular domain of CTLA-4 fused to an immunoglobulin G1 Fc region. CTLA-4 binds CD80 and CD86 on antigen-presenting cells with high affinity, outcompeting CD28 on the T cell. By occupying CD80 and CD86, abatacept prevents delivery of the co-stimulatory second signal, inducing T-cell anergy rather than depletion. This makes it a co-stimulation blocker. Anti-CD28 monoclonal antibodies are a distinct, investigational class; abatacept acts on the antigen-presenting cell side, not the T-cell side. Daratumumab, not abatacept, is the anti-CD38 agent. Rituximab, not abatacept, is the anti-CD20 agent.

Correct Answer

A — Co-stimulation blockade impairs T-cell surveillance of Epstein-Barr virus-infected B cells, and seronegative recipients lack pre-existing immunity, increasing the risk of post-transplant lymphoproliferative disorder

Rationale

Belatacept's co-stimulation blockade impairs cytotoxic T-cell control of Epstein-Barr virus-infected B cells. In Epstein-Barr virus-seropositive recipients, pre-existing T-cell memory provides a degree of residual immune surveillance even under co-stimulation blockade. Epstein-Barr virus-seronegative recipients have no pre-existing immunity; primary Epstein-Barr virus infection in the post-transplant immunosuppressed state can lead to uncontrolled Epstein-Barr virus-driven B-cell proliferation and post-transplant lymphoproliferative disorder, a potentially life-threatening B-cell lymphoproliferative disease. Epstein-Barr virus seronegativity is an absolute contraindication to belatacept use in kidney transplantation for this reason. Transplant candidates who are seronegative should receive Epstein-Barr virus vaccination when available, though belatacept remains contraindicated.

Correct Answer

C — Immune thrombocytopenia, because intravenous immunoglobulin blocks macrophage-mediated platelet destruction rather than suppressing autoantibody production

Rationale

High-dose intravenous immunoglobulin is a standard treatment for immune thrombocytopenia when a rapid platelet rise is needed — for example, before surgery or to manage active bleeding. The response is rapid (within 24 to 48 hours) precisely because intravenous immunoglobulin acts at the effector step of platelet destruction — blocking macrophage Fc-gamma receptors — rather than suppressing autoantibody production, which would take days to weeks. Aplastic anemia involves bone marrow failure, not immune platelet destruction, and intravenous immunoglobulin does not stimulate megakaryocyte proliferation. Heparin-induced thrombocytopenia is caused by platelet-activating antibodies to platelet factor 4-heparin complexes and is managed by stopping heparin, not with intravenous immunoglobulin. Thrombotic thrombocytopenic purpura results from deficient ADAMTS13 and is treated with plasma exchange, not intravenous immunoglobulin.

Correct Answer

B — Use dithiothreitol-treated reagent red blood cells or provide phenotype-matched or genotyped units to detect clinically significant alloantibodies masked by daratumumab interference

Rationale

Daratumumab binds CD38 on patient erythrocytes and on reagent red blood cells used in compatibility testing, producing a pan-reactive positive antiglobulin test that masks clinically significant alloantibodies. Two established strategies resolve this: dithiothreitol treatment of reagent red blood cells, which denatures CD38 and eliminates daratumumab binding while preserving most other blood group antigens; and extended red blood cell phenotyping or molecular genotyping of the patient (ideally performed before daratumumab is started) to provide antigen-matched units. Stopping daratumumab is not feasible around transfusions given the drug's half-life and clinical necessity. Daratumumab does interfere with compatibility testing through its coating of patient red blood cells, not donor cells — but the crossmatch uses the patient's serum against donor cells and is still affected. Empiric uncrossmatched transfusion is a last resort, not the standard approach when established solutions exist.

Correct Answer

A — Multiple myeloma and antibody-mediated transplant rejection, where the therapeutic goal is depletion of immunoglobulin-secreting plasma cells

Rationale

Bortezomib's clinical utility derives from its selective toxicity to plasma cells — the terminally differentiated immunoglobulin-secreting cells that are uniquely dependent on proteasome function. In multiple myeloma, the malignant cells are plasma cells, making bortezomib directly cytotoxic to the tumor. In antibody-mediated transplant rejection, long-lived plasma cells in the bone marrow continuously produce donor-specific antibodies; bortezomib depletes these cells to reduce antibody titers durably, addressing the limitation of plasmapheresis and intravenous immunoglobulin, which remove or block circulating antibodies without eliminating their source. Bortezomib does not selectively target T cells, does not inhibit complement activation, and is not an established therapy for rheumatoid arthritis or psoriatic arthritis.

Correct Answer

B — Pegcetacoplan blocks C3b deposition on erythrocytes, preventing splenic and hepatic macrophage phagocytosis of opsonized cells that causes extravascular hemolysis

Rationale

Eculizumab blocks C5 cleavage, preventing membrane attack complex formation and thus eliminating intravascular hemolysis in paroxysmal nocturnal hemoglobinuria. However, C3b continues to deposit on paroxysmal nocturnal hemoglobinuria erythrocytes (which lack CD55 and CD59) because eculizumab acts downstream of C3. Splenic and hepatic macrophages recognize C3b-opsonized erythrocytes through complement receptor 1, leading to phagocytosis and extravascular hemolysis — the mechanism of residual anemia. Pegcetacoplan binds C3 and C3b directly, blocking C3b deposition on erythrocyte surfaces. Without C3b opsonization, macrophages cannot recognize and phagocytose paroxysmal nocturnal hemoglobinuria erythrocytes, eliminating extravascular hemolysis and improving hemoglobin levels beyond what eculizumab achieves alone.

Correct Answer

D — Iptacopan, which inhibits factor B to selectively block the alternative complement pathway and is approved as oral monotherapy for paroxysmal nocturnal hemoglobinuria

Rationale

Iptacopan is an oral factor B inhibitor that selectively blocks the alternative complement pathway amplification loop, which is the driver of complement-mediated erythrocyte destruction in paroxysmal nocturnal hemoglobinuria. It is approved as oral monotherapy for adults with paroxysmal nocturnal hemoglobinuria, including those who are complement inhibitor-naive, offering a fully oral treatment option for patients who prefer to avoid intravenous therapy. Avacopan is an oral C5a receptor antagonist approved for anti-neutrophil cytoplasmic antibody-associated vasculitis, not paroxysmal nocturnal hemoglobinuria. Apremilast is a phosphodiesterase 4 inhibitor used in psoriasis and psoriatic arthritis; it has no role in paroxysmal nocturnal hemoglobinuria. Ozanimod is a sphingosine-1-phosphate receptor modulator used in multiple sclerosis and ulcerative colitis, with no indication in paroxysmal nocturnal hemoglobinuria.

Correct Answer

C — Abatacept induces antigen-specific T-cell anergy without broadly depleting T cells or removing tumor necrosis factor-alpha, preserving macrophage activation and granuloma integrity needed for defense against intracellular pathogens

Rationale

Tumor necrosis factor inhibitors remove a cytokine that is essential for macrophage activation and granuloma maintenance, impairing defense against intracellular pathogens such as Mycobacterium tuberculosis and requiring mandatory tuberculosis screening. Abatacept works differently: by blocking CD80 and CD86 co-stimulation, it induces anergy in antigen-activated T cells without broadly depleting them and without removing tumor necrosis factor-alpha from the system. Macrophage activation pathways, granuloma integrity, and innate immune functions remain more intact. Clinical data show that abatacept carries lower rates of serious infections and does not require mandatory tuberculosis screening in patients without latent infection risk factors, making it a reasonable alternative in patients who have experienced recurrent serious infections on tumor necrosis factor inhibitors. Abatacept does not deplete T cells and is not defined by a short half-life advantage.

Correct Answer

A — Use dithiothreitol-treated reagent red blood cells or provide phenotype-matched or genotyped blood to neutralize daratumumab interference and detect any clinically significant alloantibodies

Rationale

Daratumumab binds CD38 on patient erythrocytes, coating them with immunoglobulin G and producing a pan-reactive positive antiglobulin test that masks clinically significant alloantibodies. To safely transfuse, the blood bank must use specialized strategies: treating reagent red blood cells with dithiothreitol, which denatures CD38 and eliminates daratumumab binding while preserving most other blood group antigens; performing extended red blood cell phenotyping or genotyping of the patient before starting daratumumab; or providing antigen-matched units based on the patient's known phenotype. Transfusing the least-incompatible unit identified by standard crossmatch is not safe, as true alloantibodies capable of causing hemolytic transfusion reactions may be hidden by the pan-reactivity. Daratumumab cannot be stopped and restarted safely around transfusions; subcutaneous and intravenous formulations both achieve systemic drug concentrations and produce equivalent red blood cell interference.

Correct Answer

B — Bortezomib targets the plasma cells that produce donor-specific antibodies; because long-lived plasma cells are not depleted by plasmapheresis or intravenous immunoglobulin, bortezomib addresses the source of ongoing antibody production

Rationale

In antibody-mediated rejection, donor-specific antibodies are produced by long-lived plasma cells residing in bone marrow niches. Plasmapheresis removes circulating antibodies, and intravenous immunoglobulin blocks Fc-gamma receptors transiently, but neither depletes the plasma cells themselves; antibody titers therefore rebound as plasma cells continue secreting. Bortezomib exploits the selective vulnerability of plasma cells to proteasome inhibition — their extraordinarily high rate of immunoglobulin synthesis makes them uniquely dependent on the proteasome for misfolded protein clearance. By killing long-lived plasma cells, bortezomib targets the source of ongoing donor-specific antibody production. This plasma cell-directed approach complements — and addresses a major limitation of — antibody removal strategies alone. Bortezomib does not inhibit calcineurin, does not reduce endothelial major histocompatibility complex expression, and does not activate natural killer cells against B cells.