Immunopharmacology: Foundations and Cytokine Pharmacology

The innate immune system provides immediate, non-antigen-specific defense through neutrophils, monocytes, macrophages, dendritic cells, natural killer cells, mast cells, basophils, and eosinophils. Neutrophils are the most abundant circulating leukocytes and the first cells recruited to acute infection sites; they kill pathogens through phagocytosis, oxidative burst, and degranulation. Granulocyte colony-stimulating factor and granulocyte-macrophage colony-stimulating factor are the pharmacological regulators of neutrophil production, used clinically to reverse chemotherapy-induced neutropenia.

Macrophages are the central mediators of chronic inflammation, secreting tumor necrosis factor-alpha, interleukin-1 beta, and interleukin-6 in response to pathogen encounter. They are the primary target of tumor necrosis factor inhibitors and interleukin-1 antagonists. Dendritic cells are the most potent antigen-presenting cells and the critical link between innate sensing and adaptive activation. Abatacept and belatacept exploit the dendritic cell-to-T-cell interface by blocking the co-stimulatory ligands needed for full T-cell activation.

Natural killer cells kill virus-infected and tumor cells without prior sensitization. They also mediate antibody-dependent cellular cytotoxicity through their Fc receptor, which is how monoclonal antibodies such as rituximab and trastuzumab recruit natural killer cells to destroy opsonized targets. Calcineurin inhibitors and corticosteroids impair natural killer cell function, contributing to viral vulnerability in transplant recipients.

T lymphocytes develop in the thymus and are divided by surface markers and function into helper subsets and cytotoxic cells. CD4-positive helper T cells direct immune responses by secreting cytokines that activate macrophages, stimulate B-cell antibody production, and recruit effector cells. The major CD4 subsets are Th1 cells (producing interferon-gamma, driving macrophage activation against intracellular pathogens), Th2 cells (producing interleukin-4, interleukin-5, and interleukin-13, driving eosinophilic inflammation and IgE production), Th17 cells (producing interleukin-17A, driving neutrophilic mucosal inflammation), and regulatory T cells (producing interleukin-10 and transforming growth factor-beta, suppressing responses and maintaining self-tolerance). Drugs targeting these subsets include dupilumab (blocking the interleukin-4/interleukin-13 receptor used by Th2 cells) and secukinumab (blocking interleukin-17A produced by Th17 cells).

CD8-positive cytotoxic T lymphocytes kill target cells through perforin-granzyme pathways and Fas-mediated apoptosis. In tumor microenvironments, these cells enter functional exhaustion, upregulating checkpoint receptors including programmed death-1 and cytotoxic T-lymphocyte-associated protein 4. Checkpoint inhibitor drugs reverse this exhaustion to restore anti-tumor killing; these agents are covered in depth in Chapter 34.

B lymphocytes are the effectors of humoral immunity. Upon antigen encounter with T-cell help through the CD40 ligand-CD40 interaction, B cells proliferate, undergo class-switch recombination, and differentiate into antibody-secreting plasma cells. IgG mediates opsonization and complement activation; IgE drives mast cell and basophil degranulation in allergy. Pharmacological targeting of B cells includes rituximab and obinutuzumab (depleting B cells via anti-CD20), belimumab (reducing B-cell survival factor), and intravenous immunoglobulin (modulating Fc receptor signaling).

Tumor necrosis factor-alpha is produced primarily by activated macrophages and T cells and drives transcription of additional inflammatory cytokines, adhesion molecules, and acute-phase proteins through nuclear factor kappa B. It is central to the pathogenesis of rheumatoid arthritis, inflammatory bowel disease, psoriasis, ankylosing spondylitis, and psoriatic arthritis.

Five inhibitors are approved. Infliximab is a chimeric immunoglobulin G1 monoclonal antibody. Adalimumab and golimumab are fully human immunoglobulin G1 antibodies. Certolizumab pegol is a pegylated Fab fragment lacking the Fc region, which means it does not cross the placenta via neonatal Fc receptor-mediated transport and is the preferred tumor necrosis factor inhibitor in pregnancy. Etanercept is a dimeric fusion protein of the tumor necrosis factor receptor 2 extracellular domain with immunoglobulin G1 Fc; it binds both tumor necrosis factor-alpha and lymphotoxin-alpha and differs from the monoclonal antibodies in being ineffective in granulomatous diseases such as Crohn's disease and sarcoidosis.

Interleukin-6 is produced by macrophages, T cells, and fibroblasts and drives hepatic synthesis of acute-phase reactants, fever, Th17 differentiation, and thrombocytosis. It signals through a receptor complex including the ligand-binding interleukin-6 receptor alpha chain and the signal-transducing gp130 subunit. Tocilizumab and sarilumab are monoclonal antibodies blocking the interleukin-6 receptor alpha chain, suppressing C-reactive protein so effectively that C-reactive protein can no longer serve as an infection biomarker in patients receiving these agents. This is a critical clinical caveat: use procalcitonin and clinical assessment to detect concurrent infection in patients on interleukin-6 receptor inhibitors.

The type 2 immune response is driven by Th2 cells producing interleukin-4, interleukin-5, and interleukin-13. Interleukin-4 promotes IgE class switching and Th2 differentiation. Interleukin-5 governs eosinophil production, maturation, and survival. Interleukin-13 drives mucus hypersecretion and airway smooth muscle hyperreactivity. Dupilumab blocks the interleukin-4 receptor alpha chain shared by the receptors for both interleukin-4 and interleukin-13, producing simultaneous blockade of both cytokines; it is approved for atopic dermatitis, asthma, chronic rhinosinusitis with nasal polyposis, eosinophilic esophagitis, and prurigo nodularis. Mepolizumab and reslizumab (anti-interleukin-5) and benralizumab (anti-interleukin-5 receptor alpha) are approved for severe eosinophilic asthma.

Interleukin-23 shares its p40 subunit with interleukin-12 but pairs it with a unique p19 subunit. Interleukin-23 drives expansion of Th17 cells, which secrete interleukin-17A and interleukin-17F to stimulate neutrophilic inflammation at epithelial surfaces, driving the pathogenesis of psoriasis, psoriatic arthritis, and ankylosing spondylitis.

Ustekinumab targets the shared p40 subunit, blocking both interleukin-12 and interleukin-23, and is approved for psoriasis, psoriatic arthritis, Crohn's disease, and ulcerative colitis. Selective interleukin-23 p19 inhibitors (guselkumab, risankizumab, tildrakizumab) are more specific, preserving interleukin-12-dependent Th1 immunity while suppressing the interleukin-23/Th17 axis. Secukinumab and ixekizumab target interleukin-17A directly and carry a class-specific risk of new-onset or worsening inflammatory bowel disease; they are contraindicated in patients with active inflammatory bowel disease.