Part II Lecture on Immunoglobulins and Fc
Receptors
1st November 2011
by
Mike Clark, PhD
Reader in Therapeutic and Molecular Immunology
Department of Pathology
Division of Immunology
Cambridge University
UK CB2 1QP
http://www.antibody.me.uk/
IgG schematic
Animated Antibody Tutorial
• An excellent animated antibody tutorial is
available from Eric Martz’s website
http://www.umass.edu/molvis/bme3d/materials/jmoltuts/antibody/contents/contents.htm
Dynamic model of IgG1
Domain structure of IgG
Visit the webpage
Fc Region of human IgG1
Visit the webpage
Fab region structure
Visit the webpage
Mammalian Antibody Classes
• Antibodies are classed according to the type of
immunoglobulin heavy chain
• In addition each antibody class can have either κ
(kappa) or λ (lambda) immunoglobulin light chains
• In mammals we recognise the following classes and
chains

IgM μ

IgD
δ
(delta)

IgG
γ
(gamma)

IgA
α
(alpha)

IgE
ε
(epsilon)
(mu)
Mammalian Antibody Classes
Mammalian Antibody
subclasses
• In many species some of the classes are further subdivided
into subclasses dependent upon the existence of multiple
sub-types of immunoglobulin heavy chain
• In humans the IgG class is divided into four subclasses
IgG1, IgG2, IgG3 and IgG4, and the IgA class into IgA1
and IgA2
• In mice the IgG class is divided into IgG1, IgG2a, IgG2b
and IgG3. In some laboratory strains of mice a fifth
subclass called IgG2c has been identified although
historically this was originally classified as an allelic form
of the IgG2a subclass.
• In rats the IgG class is divided into IgG1, IgG2a, IgG2b and
IgG2c
• Despite the similarity in nomenclature the subclasses have
arisen independently in different species and thus there is
no general functional relationship between the subclasses
from each species.
Crystal structure of whole IgG2a
L.J.Harris, S.B.Larson, K.W.Hasel, A.McPherson,
"Refined structure of an intact IgG2a monoclonal antibody",
Biochemistry 36: 1581, (1997)
Schematic view of IgG domains
Kabat database variability of VH sequences
Human VH
Mouse VH
Lysozyme
CDR regions
Antibody Fv
Unlike mouse the human IgG subclasses
are very similar in sequence but they still
have different properties
Fc Receptors
• Cells of the immune system interact with antibodies using
receptors that bind to conserved structures within the constant
region domains of the immunoglobulin heavy chains, the so
called Fc region
• These receptors for antibody are thus called Fc receptors or
FcR
• Different FcR exist which show specificity for different
classes and also for different subclasses of antibody
• Of particular importance for therapeutic applications of
antibodies are those receptors that recognise IgG, the FcγR
Characteristics of human IgG Fc Receptors
FcR
CD number
Protein (Mw)
Chromosome
Transcripts
Affinity
Subunits / signaling
FcgRI
CD64
72 kDa
1q21.1
Ia, Ib, Ic
High (108-109 M-1)
3>1>4>>>2
g chain
FcgRI AGa motif
FcgRII
CD32
40 kDa
1q23-24
IIa, IIb1, IIb2,
IIb3, IIc
Low (<107 M-1)
IIa-R131: 3>1>>>2,4
IIa-H131: 3>1=2>>>4
IIa: g chain
IIa ITAM
IIb ITIM
IIc ITAM
FcgRIII
CD16
50-80 kDa
1q23-24
IIIa, IIIb
IIIa: Medium (~2x107 M-1)
IIIb: Low (< 107 M-1)
1=3>>>2,4
IIIa: g, x chain
b chain in mast
cells
aAG
= antigen presentation
Cell distribution and modulation of expression of leukocyte FcR
Regulation of expression
FcR
Cell distribution
FcgRI
Monocytes, macrophages, dendritic
cellsa, neutrophils (induced)
FcgRII
Monocytes, macrophages, dendritic
cellsa, neutrophils, eosinophils,
basophils, B cells, T cellsa, placental
endothelial cells, platelets
FcgRIII
asubpopulation
IIIa: monocytesa, macrophages, NK
cells, T cellsa
IIIb: neutrophils
Up
Down
G-CSF, IFN-g, IL-10
IL-4, IL-13
IL-4, IL-13
TGF-b
IL-4, IL-13
Human IgG Fc Receptors
CD64
CD32
CD16
Human Fcg Receptors and their Activities
IgG1=IgG3
>>IgG4
>>>IgG2
IgG1
IgG2=IgG3
>>IgG4
IgG1
IgG3
IgG4>>IgG2
FcgRI
FcgRIIa
FcgRIIb
FcgRIIc
FcgRIIIa
FcgRIIIb
(CD64)
(CD32)
(CD32)
(CD32)
(CD16)
(CD16)
a
ITAM-
Affinity
Alleles
g2
High
(108 M-1)
--
ITAM-
a
a
ITIM-
IgG1=IgG3
>>>>>>>
IgG4,IgG2
ITAM-
a
a
ITAM-
Inhibitory
Low-Med
IIA-131H
IIA-131R
HH 25%
HR 50%
RR 25%
Low-Med Low-Med
(2x106 M-1)
--
--
g2
a-GPI
Low
Low-Med
(5x105 M-1)
IIIA-158V
IIIA-158F
NA1
NA2
VV 20%
VF 40%
FF 40%
Slide courtesy of Bill Strohl, Centocor, September 2008
Cell signalling by immune complexes (1)
Activation
Cell signalling by immune complexes (2)
Inhibition
The Brambell receptor or FcRn
• This is a very important receptor with respect
to the mammalian IgG class of antibody, first
hypothesised by Brambell and thus called by
some FcRB

In adult mammals this receptor is widely
expressed, particularly on cells of the endothlium
and acts by rescuing IgG from degradation within
the endocytic pathway

The receptor is also responsible for the transport
of maternal IgG to the neonate, hence the
alternative name neonatal FcR or FcRn
The IgG receptor FcRn
Interaction with FcRn and with Protein A through similar
region
FcRn is important for IgG
half-life and transport
Effect of half-life on antibody concentration
FcRn increases the half-life of IgG to about 21 days as
compared to IgM which has a half-life of about 3 days.
Pathogen encoded HSV-1 FcR
LowgE
affinity
(59 kDa + CHO)
FcR
gI
1:1 complex forms
high-affinity FcR
Binds monomeric
IgG in 1:1 ratio
(41 kDa + CHO)
FcR function of gE and gI appears to be essential in
vivo
Binding of human IgG to cells infected with HSV-1
250
mean fluorescence
G1m(1,17)
200
G2
150
G4
uninfected control cells:
100
G1m(1,17)
50
G2
0
1
10
100
1000 10000
IgG concentration, nM
G4
Binding of human IgG1 allotype variants to cells infected with
HSV-1
160
mean fluorescence
140
120
G1m(1,17)
100
80
G1m(3)
60
G1m(null)
40
20
0
0.1
1
10
100
IgG concentration, nM
1000
Atherton, A., Armour, K.L., Bell, S., Minson,
A.C., & Clark, M.R. Eur J. Immunol. (2000)
30: 2540-547 The Herpes Simplex Virus Type 1
Fc Receptor Discriminates between IgG1
Allotypes