Gm allotypes are antigenic determinants present on the constant regions of y heavy chains which obey M endelian laws of inheritance. Since their first description by Grubb (1956 ) m any Gm allotypes have been characterised, shown in T ab le 1.3. Two systems of nomenclature exist for the classification of Gm allotypes, the World Health Organisation (W H O ) recom m ends the numeric system, although both alphabetical and numerical systems are found in the literature
Table 1.3
Gm allotypes of human immunoglobulins adapted from Lefranc andLefranc, (1990)
Subclass
Domain
Alphabetical
Numeric
IgGI
Ch3 G 1m (a) G 1m (1) Ch3 G1m (x) G 1m (2) ChI G1m (f) G 1m (3) ChI G 1m (z) G 1m (17)lgG2
Ch2 G 2m (n) G 2m (23)lgG3
Ch3 G3m (b°) G 3m (11) Ch2 G 3m (b ') G 3m (5) Ch3 G3m(b") G 3m (13) Ch2 G3m(b") G 3m (14) Ch3 G3m(b®) G 3m (10) Ch3 G3m(c^) G 3m (6) Ch3 G3m(c®) G 3m (24) Ch2 G 3m (g) G 3m (21) Ch2 G 3m (u) G 3m (26) Ch3 G3m (v) G 3m (27) Ch3 G3m (s) G 3m (15) C^3 G3m (t) G 3m (16) Ch3 G 3m (g5) G 3m (28)It is now well established that there is an association between G m allotype and serum IgG subclass levels. Individuals who are homozygous for the G 3m (b) allotype have higher lgG3 levels than G 3m (g) homozygotes. Similarly, individuals
typed as G 2m (n )’. A relationship between Gm allotype status and immune response to certain bacterial organisms, for exam ple Haem ophilus influenzae, Streptococcus pneum oniae and Meningococci, have been found. Polysaccharide specific antibody titre has been shown to be related to G 2m (n) allotype (discussed in more detail in chapter 6). It is clear that Gm allotype exerts an effect on IgG subclass concentration, although the mechanisms underlying this relationship are as yet unresolved.
S tru c tu re -F u n c tio n R e la tio n s h ip s o f th e IgG S u b c la s s e s
It is within the Fc portion of the immunoglobulin molecule that effector functions are m ediated, but there are important differences between the subclasses (Table 1.4)
T a b le 1.4 Biological and structural properties of IgG subclasses
IgGI
lgG2
IgGS
lgG4
Heavy chain
y l y 2 y 3 y 4Molecular weight of intact protein (kDa)
1 4 6 1 4 6 1 7 0 1 4 6Hinge aa length
1 5 1 2 6 2 1 2Light chain
k:X
2 . 4 1.1 1 .48
Inter-heavy chain disulphide bonds
2 4 11 2Serum levels (mg/ml)
5 - 1 2 2 - 6 0 .5 - 1 0 .2 - 1Half-life (days)
2 1 - 2 3 2 0 - 2 3 7 2 1Complement fixation
+ + + + + -Specific anti-CHO responses
+ + + + + - - /+Specific anti-protein responses
+ + - + +-/+
Protein A binding
+ + -+
Protein G binding
++
+
+
FcyRI (CD64) interaction
+++
-/+
++++
++
FcyRII (CD32) interaction
+++
+ +++++
++
C1q binding is mediated by the motif G lu 318-X -L ys320-X -L ys322. T h e context of the binding motif is of importance since the IgG subclasses differ in their ability to promote com plem ent mediated cell lysis. Human Ig G I and lgG 3 are very effective, lgG2 is effective only at high concentrations and lgG 4 is not effective (G ud dat et al. 1993; Bruggemann et al. 1987; Riechmann et al. 1988; Valim and Lachm ann, 1991). Recent studies of hinge deleted lgG3 molecules has revealed that the long flexible hinge is not itself a prerequisite for C lq binding but that the inter H chain disulphide bond is essential for com plem ent activation (Brekke et al. 1995).
Differences between the human IgG subclasses have been dem onstrated for protein A binding, with Ig G I, lgG2 and lgG 4 binding w hereas lgG 3 does not. The critical residue for protein A binding is H is435, which in the lgG 3 of Caucasians is replaced by Arg. In oriental races lgG3 having the G 3m (st) allotype is commonly found and this is associated with a histidine residue at position 4 3 5 and hence with protein A binding.
Another major effector system is the recognition of antibody coated target cells by cells expressing Fc receptors. Fc receptors m ediate a variety of functions including phagocytosis (monocytes, macrophages, neutrophils) and antibody d ependent cellular cytotoxicity (monocytes, macrophages, lymphocytes). Fey receptors fall into three categories|^cyRla,b(CD64, high affinity)^cyR lla,b (C D 32, interm ediate affinity) and FcyRllla,b ( G D I6, low affinity) have been identified in m ice and humans (Anderson and Looney, 1986) . FcyRI binds m onomeric IgG w hereas FcyRII and III only bind aggregated IgG in the form of immune com plexes. The three FcyRs are probably derived from a common ancestral
g en e and are m embers of the Ig gene superfamily (Fridman et al. 1992). FcyR binding has been shown to be mediated by the extrem e N-terminal portion of the Ch2 domain. In particular, Leu235 has been demonstrated to be a critical residue in such binding (W oof et al. 1986; Canfield and Morrison, 1991).
Th e human IgG subclasses differ in their ability to bind Fc receptors. Hum an FcyRI is expressed on monocytes and m acrophages and its expression on neutrophils may be induced by IFNy in vitro (Perussia et al. 1983). The binding of the IgG subclasses differ with lgG3 exhibiting the highest affinity (Burton, 1985). lgG2 is thought to be incapable of binding to FcyRI (W alker et al. 1989), but m ay bind with very low affinity (Van Den Herik Oudijk et al. 1994). FcyRII is expressed on virtually all haematopoetic cells, w hereas FcyRIII expression is restricted to m acrophages, NK and m ast cells (W einshank et al.
1988; Daeron et al. 1990).
FcyR m ediate a wide spectrum of activities, related to cell type. For exam p le receptors expressed by macrophages m ediate phagocytosis, endocytosis, antigen presentation and killing of IgG sensitised cells, a function also shared with NK cells (Fridman et al. 1992).