Hyper IgM and Immunodeficiencies

Hyper IgM Mutations

• Mostly pathways of T cell help
• Excess of IgM and deficiency of IgG and IgA
• Failure to provide T cell help causes immunodeficiency
• Some bacterial antigens don't require T cell help due to repetitive nature and TLR agonists which stimulate B cells to proliferate and differentiate into IgM secreting cells.

X-linked CD40 Ligand Deficiency (HIGM1)

• X-linked, more frequent in males
• Combined immunodeficiency: defects in antibody production and cellular immunity and is therefore more severe.
• Defective B cell proliferation.
• No germinal centers or immunological memory.

CD40 Deficiency

• Autosomal recessive, hyper IgM syndrome
• Humoral immune deficiency, no germinal centers, and no immune memory
• Cellular immunity is largely intact
• Healthy individual: CD40 ligand engages CD40
• Individual lacking CD40 ligand: no engagement of CD40, limited proliferation and differentiation resulting in only IgM production

AID (AICDA) Deficiency

• Autosomal recessive
• Germinal centers form, but no class switch recombination, no class switch memory, and no somatic hypermutation
• Failure to class switch results in only IgM production
• Inability to make IgG and IgA creates severe immune defects

AID C-terminus Mutations

• Germinal centers form, no class switch recombination, but somatic mutation occurs
• Continue to make only IgM, but with improved affinity
• Effective functions are still missing and important

UNG Deficiency

• UNG removes uracils created by AID activity on DNA that forms somatic hypermutation
• Acts like an AID deficiency with no class switch recombination and no somatic mutation

Therapy for Hyper IgM

• Typically IVIG (intravenous immunoglobulin)
• In severe cases of X-linked hyper IgM, bone marrow transplantation is attempted

Common Variable Immunodeficiency (CVID)

• Vast majority of immunodeficiencies are in the category of CVID
• Challenging collection of syndromes and symptoms
• Very heterogeneous, lacking uniform criteria for diagnosis
• Characterized by reduced immunoglobulin, increased susceptibility to recurrent infection, and poor vaccine responses
• Genetic defect identified in about 30% of cases, while the basis remains unclear in up to 70%
• Diagnosis of exclusion, often after patients suffer debilitating infections

Criteria for CVID Diagnosis

• Hypogammaglobulinemia (reduced serum immunoglobulin)
• Recurrent bacterial infections of the same bacterium
• Reduced antibody production in response to vaccination (several fold lower)
• There are two waves of diagnosis around five years old and around twenty years old.

Challenges in Diagnosis:

• Difficulty to diagnose in young children due to immunological immaturity
• Delay in diagnosis: 8-10 years after the first symptom
• Delay in diagnosis of one year for every year is associated with a 4% increased risk of mortality

Treatment for CVID:

• Monthly infusions of IVIG (intravenous immunoglobulin)
• About 50% of all IVIG usage in Australia is associated with treating patients with CVID
• CVID patients are protected from infection by IVIG, but some still develop severe complications of autoimmunity, cancer, and lung disease

Mutations Causative of CVID

• Many mutations contribute to CVID
• Small contribution/incidence of each mutation
• Many mutations recognized from B cell development and activation schema

Examples:

• Activation receptors: CD19, CR2, CD81
• TNF SF12
• Signaling molecules: NF kappa B, protein kinase C delta, phospholipase C gamma two, B lymphocyte kinase, VAV2 (VAV guanine nucleotide exchange factor)
• Negative regulators: LRBA, promotes CTLA-four expression, PIK3CD causes constitutive activation within the PI3 kinase pathway
  • LRBA mutations: T cell hyperactivity, premature death, deficiency of functional T cells
  • PIK3CD mutations: constitutive activation, cells inappropriately kill themselves, immune insufficiency
• Genes modifying lymphocyte behavior: Tasi and BAF receptor, regulate B cell survival and activation.

CVID Pathophysiology

• Multiple pathways can be mutated, affecting function without affecting development (normal lymphocyte numbers)
• Partial block rather than absolute block
• Severity progresses over time, making diagnosis difficult
• Multiple mutations in different pathways collectively lead to immune deficiency

Primary Antibody and Combined Immune Deficiencies

• Mutations that block B cell development cause immune deficiencies (e.g., XLA and BTK mutation)
• Common treatment are IVIG for the most part.
• Relationship of B cell signaling with development and differentiation: issues with B cell signaling can result in deficiencies in both B cell production and function
• CVID complexity and known causative factors
• Challenges in treating CVID

B Cell and T Cell Interactions

• B cells depend substantially on instructions and stimulation from T cells

Molecular Interactions

• Extensive interactions between B cells and T cells are required for immune responses
• Complexity of signal transduction processes downstream from interacting molecules
• CVID can arise from absence of interacting molecules or mutations in downstream genes
• UNG and PMS2 are involved in processes triggered by AID (class switch recombination and somatic mutation)
• NEMO: NfB essential modulator, required for NF kappa B activation.
• ICOS: inducible T cell co-stimulator, interacts with ICOSL on B cell and in absence of ICOS, T cells will do little to help B cells and so B cell antibody production will be diminished.

T Cell Activation of B Cells

• Activated T cell sees antigen and co-stimulation on a B cell
• B cell presents antigen on MHC II molecules to T cell
• T cell expresses ICOS, binds ICOS ligand on B cell
• T cell upregulates CD40 ligand, binds CD40 on B cell, leads to B cell proliferation

Cytokine Secretion and B Cell Response

• T cell secretes cytokines (e.g., interleukin 21 from T follicular helper cell)
• B cells receive CD40 ligand and IL21, triggering proliferation, AID, UNG, PMS2 induction, and NF kappa B activation
• Absence of T cell help leads to defective antibody production, failure to class switch recombination, and failure to affinity maturation

Red-colored mutations identified as immune deficiencies:

• From this we can see ICOS is CVID1, we have NF kappa b 1 and 2, failure to produce interleukin 21, AR CVID1, CD40 ligand mutation on the X chromosome hence XL CD40l defective, CD40, autosomal recessive, R21 receptor deficiency and there are two forms autosomal recessive and autosomal dominant, mutation in AID which makes the patient hyper IgM.

Therapeutic Options: IVIG

• Delivery of antibody in IVIG format
• Universal treatment for primary antibody deficiencies like XLA
• Intravenous immunoglobulin (Intrigam from CSL)
• 10% weight for volume solution of IgG purified from pooled sera of ~1000 blood donors
• Contains all IgG subclasses with traces of IgM and IgA
• Dose: 400-600 mg/kg/month, infused over 2-4 hours
• Maintains IgG at ~5 g/L at trough (minimum concentration before next dose)
• Normal IgG: 12-13 g/L, half-life ~21 days
• Cost: ~$100,000/year per patient, but life-saving

Effectiveness of IVIG:

• Very successful, prolongs life but does not eliminate infections
• Broad-spectrum infectious agent treatment due to transfer of protective antibodies from the community
• Specific IVIG types available enriched for VZV and CMV

Alternative Treatment: Subcutaneous Immunoglobulin (SCIG)

• Administered by patient themselves
• More frequent dosing (every two weeks)
• Smaller dose (100-150 mg/kg in 40-60 mL)
• Delivered via syringe or small pump in abdomen, thigh, or forearms
• Better preventative activity than IVIG due to avoiding peaks and troughs
• More expensive than IVIG, but potentially more effective and avoids clinical time/supervision

Quality of Life for XLA Patients on IVIG

• Hospitalized more frequently and missed work/school, but quality of life generally equivalent to other adults
• achieved higher levels of education and had a higher income than did the general population (sociological phenomenon)

Alternate Treatments for XLA

• Stem cell transplantation previously had no beneficial effect, but becoming more efficient and safer
• Gene therapy opportunities improving as mutations are better identified, particularly for white blood cell system
• Potential for stem cell recovery, stimulation, gene introduction, and reintroduction into recipient
• Balancing efficacy, effectiveness, risk, cost, and alternatives

Mutations Causing B Cell Insufficiency

Key B Cell Mutation

• Most common: Bruton's tyrosine kinase deficiency (BTK), 1 in 200,000 live births, X-linked (males)
• Mutations prevent expression/activity of BTK, important signal transduction molecule downstream of B cell receptor
• Normal pro-B cells, but blocked beyond that due to inability to signal successful rearrangement
• BTK deficiency accounts for ~85% of Agammaglobulinemia cases

Other B Cell Mutations

• Mutations in IgM heavy chain: inability to express mu heavy chain and block of B cell development
• Lambda 5 deficiency: blocks B cell development at pre-B cell stage due to failure to signal successful heavy chain rearrangement
• Ig alpha/beta deficiency: absence of signaling molecules associated with B cell receptor, B cells are ineffective
• BLNK deficiency: signaling molecule downstream of B cell receptor, blocks signaling and B cell production
• LRRC8 deficiency: critical role in B cell development, no B cells produced
• PI3K deficiency: key component downstream of immunoglobulin receptor, ineffective signaling
  *PIK3R1 actually a negative regulator of PI3 kinase. In its absence PI3 kinase is effectively overactive and that results in its negative regulation.

B Cell Receptor (BCR) Signaling Complex

• Immunoglobulin heavy chain combined with surrogate light chain

• Associated with IgA and beta, which have extracellular and intracellular domains

• Signaling complex assembles around intracellular domains of Ig alpha/beta to transmit signals through BCR

• Circled mutations in the scheme:

• Surrogate light chain: absence prevents molecule from reaching the surface

• Ig alpha/beta: absence prevents signaling scaffold formation

• Bruton's tyrosine kinase (BTK): key mediator of signaling, absence stops transmission

• BLNK: target of BTK, absence stops signaling

• These mutations block signals through pre-B cell receptor and block the progression of B cell development

  X-Linked A gamma globulinemia (XLA) and BTK:

• XLA is X linked A gamma globulinemia without B cells which is caused by a mutation in the BTK gene on the X chromosome.

Immunodeficiencies

• Process of immune responses are arrested, inappropriate, or unavailable
• Individual is susceptible to immune failure and infections
• Primary antibody and combined immunodeficiencies
• Specific mutations in genes and defects in proteins block B cell development and cause immune deficiencies

Primary vs. Secondary Immunodeficiencies

• Primary immunodeficiency: intrinsic and not caused by other diseases, treatments, or environmental agents
• Secondary immunodeficiency: caused by other factors

Types of Primary Immunodeficiencies:

• Antibody deficiencies: loss of some or all of humoral immunity, cellular immunity intact
• Associated with absent/reduced or normal but non-functional B cells due to developmental defects
• CVID (Common Variable Immunodeficiency): typically differentiation defects, lymphocytes present but fail to respond appropriately
• Hyper IgM syndromes: B cells present, can respond to antigenic challenge, but unable to differentiate appropriately (stuck making IgM)
  Combined Immune Deficiencies (CID):
  -loss of humoral and cellular immunity.

Combined Immune Deficiencies (CID)

• Includes Severe Combined Immune Deficiencies (SCID)
• Combined loss of humoral and cellular immunity
• Failure in T cell activation and linked failure in B cell activation (due to dependence on CD4 T cells)
• T cell immune deficiencies are generally more severe than B cell deficiencies
• Humoral deficiencies have therapeutic options enabling relatively normal lifespans

Primary Immune Deficiencies: Outcomes

• Result of inborn errors of immune development
• Associated with autoimmunity, susceptibility to infections, cancer, or combinations thereof
• Severe, typically result of single mutations
• CVID can have multiple contributing mutations, many unidentified

David Vetter: "The Boy in the Bubble"

• Lived from 1971 to 1985 in a sterile plastic bubble due to immunodeficiency
• Chamber built by NASA.
• Lacked T cells completely due to mutation in a single gene
• * No adaptive immunity, unable to activate B cells or mediate cellular immunity
• Incredibly susceptible to lethal infection

Major Types of Severe Combined Immune Deficiency (SCID)

Adenosine Deaminase (ADA) Deficiency

• Most common but still rare
• Mutations in gamma and coma chain of IL-two receptor is critical for T cell development.
• Deficient in T cells and NK cells

JAK3 Kinase Deficiency

• Also deficient in T cells and NK cells; B cells present

IL-7 Receptor Deficiency

• Loss of T cells, as IL-7 is a growth factor in T cell development

Loss of RAG1, RAG2

• Artemis is a component of the DNA repair process that occurs in lymphocytes after the cleavage of DNA by Rag they are the enzymes that are responsible for cleaving the DNA to permit variable region gene rearrangement
• No T cells, no B cells

Potential Therapies

• Hematopoietic stem cell transplantation (HSCT)
• Specific gene therapy attempts (e.g., ADA deficiency)