Basic science of B cell disorders

Stuart Tangye

Human B-cell development and differentiation

Human B-cell development and differentiationMutations compromising B cell development

- agammaglobulinemia• BTK (XLA)

• Ig µ, Igα, Igβ• λ5

• BLNK• E47, IKAROS

(• RAG1/2)X X X

X X X X

X X

Kinetics of the Humoral Immune Response

B-Cell Activation in Lymphoid Tissues

1. Initiation of 1˚ response in the T cell zone

“Extrafollicular reaction”

Short lived cells

Kinetics of the Humoral Immune Response

“Extrafollicular reaction”

B-Cell Activation in Lymphoid Tissues2. Maturation of the B cell response: “Germinal centre” reaction

Periph blood, 2˚ lymph tissues, BM

Bone Marrow

Dark Zone-  centroblasts -  SHM-  intense proliferation

Light Zone- centrocytes-  hi-affinity clones receive survival

signals, express survival genes-  Recycle to DZ, or exit

Outer Zone- differentiation of hi-affinity centrocytes into memory B or plasma cells

De Silva & Klein (2015). Nat Rev Immunol 15: 137

Kinetics of the Humoral Immune Response

“GC reaction”

Why mutate Ig V region genes?

• allows immune system to select B cells expressing Ag receptors (ie Ig) with increased/improved affinity for the immunising Ag/pathogen

• generates highly Ag-specific immune response

• provided B cells with a survival advantage

• facilitates more rapid elimination of infectious pathogen

Why switch Ig isotypes?

• isotype switching provides secreted Ig with distinct biological activities

- IgG : longer t1/2 than IgM; : binds FcγR to increase phagocytosis: activates complement pathway/system

- IgA : produced predominantly in the mucosa: often 1st line of defence against pathogens: dimeric

- IgE : binds IgεR to activate mast cells, basophils: important in anti-helminth responses

• necessary for adaptive Ab responses

IgM IgD IgG IgA

CSR & SHM: AID-dependent processes

Delkeret al., 2010. Nat Immunol. 10: 1147

GC and memory formation are T-cell dependent

• nude mice (greatly deteriorated thymus)• thymectomised mice• anti-CD4 mAb treated mice• αβ TCR gene deficient mice

ALL UNABLE TO MOUNT LONG LIVED Ab RESPONSES

Control mice

Thymectomised mice

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But which molecules, interactions and cells are required?

Role of CD40 During a B-Cell Response

Van Kooten & Banchereau (1996). Adv. Immunol. 61: 1-77

IL-21 IL-21

IL-21

IL-21

What would happen if the CD40/CD40L interaction was prevented?

x

Van Kooten & Banchereau (1996). Adv. Immunol. 61: 1-77

x

x

x

IL-21 IL-21

IL-21

IL-21

Hyper-IgM Syndrome (Ig class switch recombination deficiencies)

•  ↑ or normal IgM •  ↓ IgG ↓ IgA ↓ IgE •  Ie impaired CSR; reduced SHM •  Extremely susceptible to recurrent bacterial and

opportunistic infections –  Pneumocystis carinii pneumonia, –  cryptosporidium–  CMV

•  No germinal centers (ie TD immune responses) •  Lack of humoral memory •  X-lined and autosomal forms

X-lined Hyper-IgM Syndrome (HIGM-1)

•  ~50% of all cases of HIGM (TRAP = CD40L)

Defective GC formation in HIGM1Control HIGM

H&E

Ki67

CD23 (FDCs)

Facchetti et al., 1995. J Immunol. 154: 6624

Autosomal recessive HIGM: HIGM3

Engagement of CD40 activates NFκB

Puel et al. 2004. Curr. Opin. Immunol. 16: 34

Lack of T-cell help

Unable to receive or transmit T-cell help

Durandy & Honjo (2001). Curr Opin Immunol 13: 543

• reduced IgM memory B cells, no switched memory B cells

Hyper-IgM Syndrome

B-cell intrinsic HIGM: mutations in mediators of CSR and SHM

B-cell intrinsic HIGM: HIGM2•  ↑ or normal IgM •  ↓ IgG ↓ IgA ↓ IgE •  impaired CSR (± SHM) •  Hyperplastic lymph nodes/germinal centers •  Intact T cell function g not susceptible to opportunistic infections •  Often not recognised til 2nd or 3rd decade of life

Revy et al (2000). Cell 102: 565

B-cell intrinsic HIGM: HIGM2•  ↑ or normal IgM •  ↓ IgG ↓ IgA ↓ IgE •  Impaired CSR (± SHM) •  Hyperplastic lymph nodes/germinal centers •  Intact T cell function g not susceptible to opportunistic infections •  Often not recognised til 2nd or 3rd decade of life

Clinical/immunologic features of HIGM syndromes

Notarangelo et al., 2006. J Allergy Clin Immunol. 117: 855

CD40L is predominantly expressed by CD4+ T cells in the outer zone of GC’s (Tfh cells)

Hutloff et al (1999). Nature. 397: 263

…..as is ICOS Casamayor-Palleja et al (1995). J Exp Med. 181: 1293

CD40L Ki67 CD4

Inducible Co-Stimulator (ICOS)

Schwartz (2001). Nature. 409: 31

4/36 CVID patients

IL-21R/γc/JAK3/STAT3 signalling is necessary for humoral immunity in vivo in humans

STAT3JAK3

T follicular helper (TFH) cells• CD4+ CXCR5hi T cells that localise to B-cell follicles/germinal centres• express high levels of many molecules involved in T/B interaction/regulation

• provide help to B cells via production of specific cytokines

Ma et al. 2009. Immunol Cell Biol. 87: 590

X-HIGM ICOS CVID XLP

Correlation between TFH and memory B cells

Intrinsic and extrinsic regulators of human B-cell differentiation

B-cell differentiation: the role of co-receptors

Rolink & Melchers (2002). Curr Opin Immunol. 14: 266

CD19: B-cell co-receptorOpsonised Ag - ligates 2 receptors

CD19-deficient patients

CD19-deficient patients

-  6 patients total

-  4 unrelated families; different homozygous mutations

-  recurrent infection, hypogamma, impaired recall responses to vaccine Ags

-  Memory B cell deficiency

-  normal GC formation, reduced SHM

-  reduced B cell responses in vitro

-  (2 other unrelated CD19 mutant pts been reported; initially diagnosed as B-cell deficient due to lack of CD19+ cells!)

Another case of CD19 deficiency?

Another case of CD19 deficiency?

-  phenocopy of CD19-deficiency

CD21 deficiency

-  hypogamma ± recurrent infections, reduced memory B cells, reduced response to polysacch Ag; ± defective B cell responses in vitro

-  not as severe as CD19-deficiency

Rolink & Melchers (2002). Curr Opin Immunol. 14: 266

-  reduced switched memory B cells

-  low serum IgG

-  impaired responses to TI Ag (ie PneumoVax) but not TD Ag

B-cell differentiation: the role of co-receptors

Novel gene mutations underlying CVID and humoral immune defects

Novel gene mutations underlying CVID and humoral immune defects

NFKB2 (p100)

NFKB1 (p50)

B-cell differentiation: the role of co-receptors

Rolink & Melchers (2002). Curr Opin Immunol. 14: 266

Intrinsic and extrinsic regulators of human B-cell differentiation