Lecture 24 - Immunodeficiency

primary (congenital/inherited) immunodeficiency
- innate immune deficiencies
  - leukocyte adhesion deficiency
    - defects in integrins or other molecules involved in cell extravasation out of blood vessels
    - Pathophysiology: Mutations in integrins or related adhesion/cytoskeletal components disrupt selectin-glycoprotein rolling, integrin–ICAM firm adhesion, and transmigration (extravasation), impairing neutrophil slowing and squeezing between endothelial cells
    - Clinical impact: Neutrophils (and monocyte-derived macrophages) fail to reach infection sites, limiting inflammatory cell recruitment; macrophages may still arrive, but neutrophil absence is prominent
  - chronic granulomatous disease
    - Molecular defect: Mutations in components of the phagocyte oxidase (NADPH oxidase/“phox”) complex reduce reactive oxygen species (ROS) generation in phagolysosomes.
    - Reactive nitrogen species interplay: Nitric oxide (NO) production remains intact, but ROS deficiency prevents formation of combined reactive nitrogen species (e.g., peroxynitrite, ONOO−), reducing microbicidal capacity.
    - Susceptibility: Increased risk for bacterial and fungal infections, including opportunists such as Aspergillus fumigatus, due to impaired intracellular killing after phagocytosis
- B and/or T cell deficiencies
  - severe combined immunodeficiency (SCID)
    - individuals that lack both T and B cells
    - Definition: Severe combined immunodeficiency; mutations in recombination/repair genes lead to absent T and B cells and loss of adaptive immunity
    - Clinical consequence: Profound susceptibility to bacterial, viral, fungal, and other infections; innate immunity is insufficient to compensate
  - agammaglobulinemia
    - functional T cells but defects in B cell development
    - Concept: Absence of immunoglobulins due to failed B-cell development while T cells may remain functional
    - Infection profile: Greater susceptibility to bacteria, fungi, and helminths (processes requiring antibodies for neutralization, opsonization, degranulation); viral infections are less impacted relative to bacterial/fungal when T cells are intact, though neutralizing antibodies also aid in viral control
  - defects in T cell development and maturation
  - defects in T and/or B cell activation
  - defects in genes specific to T cell development also inhibit B cell-mediated immunity
    - TCR complex signaling
      - hyper IgM syndrome — defects in B cell instrinic activation of isotype switching (AID required)
        Absence of T cells or defective CD40–CD40L interaction prevents isotype switching and germinal center reactions
        AID deficiency blocks somatic hypermutation and class switch recombination, locking antibody production at IgM
        Downstream signaling defects (e.g., NEMO in NF-κB pathway) can similarly impair switching
        outcome: elevated IgM with paucity of switched isotypes (IgG, IgA, IgE), leading to impaired responses to many pathogens
      - defects in genes involved in T cell-mediated activation of B cells
        CD40/CD40L (also important for Th1 activation of macrophages)
      - defects in CD8+ T cell induction of apoptosis
        Perforin mutations: Even with T cells present, perforin defects abrogate cytotoxic granule-mediated apoptosis of infected cells, impairing CTL responses
        Macrophage activation: T helper cells are required for activating macrophages to kill intracellular microbes; T-cell deficiencies compromise macrophage control of infections
  - includes mutations of genes required for VDJ recombination
    - RAG1/2 - recombination activating genes
    - Artemis complex - component of recombination machinery, associated with RAD50 complex
    - DNA ligase 4 - required for sealing DNA breaks via phosphodiester bond formation
    - DNA-PKcs - crucial for DNA repair during recombination
T cell-development and -independent activation revisit
- T cell-independent activation require multivalent binding
  - Signal 1: BCR binding antigen.
  - Signal 2: Innate co-receptors (e.g., PRRs or complement receptors) provide costimulation.
  - Signal 3: T-cell help via CD40–CD40L and cytokines leading to germinal center reactions.
- does not allow for somatic hypermutation and affinity maturation, isotype switching, and memory formation
- Downstream processes: Cyclin D1/myc-driven proliferation; AID-mediated somatic hypermutation and isotype switching; differentiation into plasma cells and memory B cells
- T cell–independent activation: B cells can be activated without T-cell help but do not undergo somatic hypermutation, isotype switching, or memory formation; output is predominantly IgM
treatments for primary immunodeficiencies
- hematopoietic stem cell transplant
  - addresses root causes by reconstituting hematopoiesis; bone marrow transplant for T- and B-cell developmental defects
- intravenous Ig
- gene therapy
- gene editing
  - CRISPR used to correct specific mutations in DNA associated with immunodeficiencies
- passive immunity
  - administration of antibodies (e.g., IVIG) to compensate for humoral defects; feasible even when T-cell transfer is not practical

secondary (acquired) immunodeficiency treatments
- chemotherapy for cancer by myelosuppression affecting precursors for all blood cells, reducing leukocyte development, resulting in immunodeficiency
- bone marrow cancers (metastasis & leukemia) occupy/reduce marrow niches and diminish leukocyte development capacity
- protein-calorie malnutrition energy-intensive lymphocyte development (recombination, selection) falters without adequate nutrition; traditional admonitions to “eat well” reflect genuine immunologic needs
- splenectomy
  - Phagocytosis impact: Loss of a major macrophage reservoir diminishes blood-borne pathogen clearance
  - Filtering role: Spleen filters blood, catching pathogens; removal impairs pathogen capture
  - B-cell maturation: Spleen houses B-cell maturation; removal reduces capacity for efficient antibody responses
- immunosuppressive for graft rejection and inflammatory diseases
  - primarily used for organ/cell transplantation or autoimmune disease
  - historically small molecule inhibitors
    - glucocorticoids/corticosteroids
      - nature and entry: Steroidal lipids traverse membranes easily; bind intracellular glucocorticoid receptor (GR), the receptor for cortisol, the stress hormone
      - nuclear actions: GR translocates to nucleus; binds glucocorticoid response elements (GREs) to activate/suppress target genes; can also bind NF-κB (and other transcription factors) to inhibit its DNA binding, suppressing pro-inflammatory gene expression
      - outcome: Broad repression of NF-κB–target genes and inflammatory cytokines; metabolic effects also noted
      - mimics of the endogenous hormone cortisol “stress hormone”
      - affects metabolism and inflammation
      - bind to glucocorticoid receptor (GR) — directly bind to promoters in DNA to activate expression of anti-inflammatory genes
      - bind to other transcription factor complexes to inhibit activity
        tran-repression
        NF-kB transcription factor is a targe
      - activates expression of regulators of mRNA stability
        decreases protein expression of cytokines like TNF-a
    - calcineurin inhibitors such as cyclosporin & tacrolimus
  - biologics
    - anti-TNF agents: mAbs bind TNF-α, preventing receptor engagement and NF-κB activation
    - IL-1 pathway blockers:
      - anakinra (Kineret): mimics IL-1 structure, binds IL-1 receptor to block activation (receptor antagonism)
      - Anti–IL-1β monoclonal antibodies: Neutralize IL-1β directly.
    - IL-6 pathway blockers:
      - target IL-6 receptor (membrane-bound or soluble forms); biomolecules bind and neutralize receptor signaling regardless of receptor form, preventing downstream activation
    - biopharmaceutical drug products synthesized in or extracted from a living organism such as insulin, vaccines, and engineered mAbs that are structurally resembling biological components and administered by injection
      - functions of Ab — neutralization, agglutination, ADCC, degranulation and opsonization
    - infliximab
    - targeted suppression of specific pathways
      - block TNF-a signaling by engineered mAbs that bind TNF-a
      - block IL-1B signaling with anakinra (an antagonist that binds to IL-1 receptor) & canakinumab (a mAb that binds to IL-1B)
      - block IL-6 signaling with tocilizumab (an antagonist that binds to IL-6 receptor)
  - JAK inhibitors
    - block all JAK-STAT signaling downstream of multiple receptors
    - Mechanism: Many cytokines signal via JAK-STAT; inhibitors block intracellular kinases, preventing transcriptional responses even when cytokine and receptor are intact
    - Positioning: Acts downstream of receptor engagement; complements extracellular neutralization strategies
    - Signaling cascade summary: cytokines released bind receptors on target cells (macrophages, B cells, T cells, etc.)
      - activation triggers NF-κB or JAK-STAT pathways
      - drugs act by blocking ligand binding, receptor function, or intracellular signal transduction to reduce inflammation and immune activation
- inhibit TCR signaling and downstream T cell activation by:
  - calcineurin inhibitors such as cyclosporin, tacrolimus and glucocorticoids
  - inhibit downstream activation of IL-2 signaling with mTOR inhibitors and cell cycle inhibitors
- receptor activation to nucleus: TCR/PRRs activate intracellular cascades leading to NF-κB and other transcription factors, initiating expression of inflammatory cytokines (e.g., TNF-α, IL-1β, IL-6)
- drug action points: Suppress NF-κB activity or downstream gene expression; block cytokine–receptor interactions; inhibit JAK-STAT signaling
revisit to NF-kB regulated pro-inflammatory cytokines
- IL-1B, TNF-a, IL-6, CXCL8, IL-12
- inflammation: redness, swelling, pain, fever

secondary (acquired) immunodeficiency
- human immunodeficiency virus infection — HIV/AIDS
  - a retrovirus with RNA genome and reverse transcriptase enzyme which synthesizes a DNA intermediate
  - RNA-dependent DNA polymerases have no proofreading = high rate of mutation
  - HIV specifically infects immune cells
    - entry into cells depends on binding of virus to CD4+ and a chemokine receptor (CCR5 or CXC4)
    - viral replication depends on activation of host NF-kB TF
    - cells must be activated by cytokines and/or PAMP-PRR signaling
  - mutations in CCR5 cause resistance to HIV
    - CCR5-Δ32
      - 32 bp deletion that causes frameshift mutations & causes premature stop codons
      - makes receptor non-functional, HIV cannot bind
    - “Berlin patient” in 2008
      - HIV positive & had acute myeloid leukemia
      - treated with hematopoetic stem cell transplant
        used stem cells from a donor homozygous for Δ32 mutation
      - patient now has no more detectable virus
      - process repeated for “London patient” in 2019
    - in 2018, a scientist in China genetically edited human embryos with CRISPR to mutate CCR5
      - not ethical!
        many current anti-retroviral drugs are very effective against HIV and mutations in CCR5 can cause more susceptibility to other infections

Anti-retroviral drugs

elite controllers
- a subset of infected people are able to suppress HIV replications for decades
  - “long-term nonprogressors”
- multiple molecular mechanisms:
  - specific MHC allotypes are better able to present HIV peptides and stimulate strong cytotoxic T cell responses
  - specific alleles of the NK cell receptors KIR also promote NK cell activation and control HIV
  - broadly neutralizing Ab formed from multiple rounds of somatic hypermutation bind conserved and required portions of the HIV envelope glycoprotein