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Tryptic cleavage of rat IgG: A comparative study between subclasses
Immunology Letters, 3 (1981) 93-98
Elsevier/North-HollandBiomedicalPress
T R Y P T I C C L E A V A G E O F R A T lgG: A C O M P A R A T I V E S T U D Y B E T W E E N S U B C L A S S E S J. ROUSSEAUX, R. ROUSSEAUX-PREVOST, H. BAZIN* and G. BISERTE Institut de Reeherches sur le Cancer, INSERM U 124, BP 311, 59020 Lille Cddex, France and *Experimental Immunology Unit, Faculty of Medicine, University of Louvain, Clos-Chapelle-aux-Champs, 30-1200 Brussels, Belgium
(Received 15 September 1980) (Revised version received 13 February 1981) (Accepted 13 February 1981)
1. Summary Monoclonal rat IgG belonging to the 4 rat IgG subclasses, and some IgG subclasses isolated from normal rat serum were subjected to enzymatic degradation with trypsin. Differences in the products of tryptic digestion were observed according to the IgG subclass. IgG2b and IgG2c were degraded mainly into Fab and Fc fragments. IgG 1 and IgG2a appeared resistant to such cleavage. However, tryptic digestion of the latter two produced two fragments separated only in dissociating media. Results of the studies suggest that one of the fragments (mol. wt. 13,000) probably consists of most of the variable domain of the 3' heavy chain, while the second (mol. wt. 120,000) consists of the IgG deleted of the VH regions.
sin than IgG2a and IgG2b: these latter two are split into Fab- and Fc-like fragments that are not found in the digests of IgG 1. In the rat, Nezlin et al. [3] have demonstrated a different susceptibility to trypsin of two IgG subclasses: IgG2a and IgG2b. IgG2a is resistant to the enzyme and left undigested whereas IgG2b is degraded into Fab and Fc fragments. The availability of the monoclonal IgG secreted by the LOU rat immunocytomas [4,5] led us to extend the study to a comparison of the 4 rat IgG subclasses: IgG1, IgG2a, IgG2b and IgG2c [6]. Our results showed that IgG2b and IgG2c are susceptible to trypsin and split mainly into Fab. and Fc-like fragments. On the contrary, IgG 1 and IgG2a appeared mainly unsplit when studied in non-dissociating conditions, but were separated in dissociating media into two components, one of which is probably related to most of the VH domain and the other consists in the IgG depleted of VH.
2. Introduction Enzymatic hydrolysis has been used for several years as a means for immunochemical analysis of the immunoglobulins, in particular for the characterization of the subclasses. It has been shown that IgG subclasses differ in their sensitivity to the proteolytic enzymes, especially papain and pepsin. The action of trypsin has also been studied but less extensively. Human IgG1, IgG 3 and IgG4 are susceptible to trypsin cleavage, which produces Fab- and Fc-like fragments: tFab and tFc [ 1]. On the other hand, IgG 2 is resistant to trypsin and fragmentation occurs only after long times of digestion. In the mouse, studies by Gorini et al. [2] have shown that IgG 1 is more resistant to tryp-
3. Materials and methods 3.1. l m m u n o c y t o m a s Immunocytomas producing monoclonal immunoglobulins were maintained in our laboratory as described previously [4]. 3.2. Purification o f rat immunoglobulins
Normal rat serum was obtained from randomly bred Wistar rats. Monoclonal rat IgG and IgG2a from normal rat serum were isolated as described previously [7]. IgG2c from normal rat serum was purified by chromatog-
0165-2478/81/0000-0000/$2.50 © Elsevier/North-HollandBiomedicalPress
93
raphy of a precipitate at 40% ammonium sulphate saturation on a protein A-Sepharose (Pharmacia, Uppsala, Sweden) column. Elution was performed sequentially with 0.01 M sodium phosphate, 0.15 M NaC1 pH 8.0, 0.1 M sodium citrate, pH 6.0 and pH 3.0. Fraction eluted at pH 3.0 contains only IgG2c and represents about 90% of IgG2c from normal rat serum [81. 3.3. Antisera Polyvalent rabbit antisera to normal rat serum proreins were purchased from Sebia. Antisera specific for the heavy chain of rat IgG were prepared as described previously [6].
3.4. Enzyma tic digestions TPCK-trypsin was purchased from Worthington Biochemicals. Mercuripapain (25 mg/ml) and twice crystallized pepsin were from Sigma Chemicals. Tryptic digestions were performed as follows: rat IgG (10 mg/ml) in 0.1 M Tris • HC1 buffer, pH 7.8, containing 0.02 M CaC12, were digested for 4 h or 18 h with an enzyme:protein ratio of 2% (w/w) with or without the addition of 0.01 M cysteine. The digestion was terminated by addition of soybean trypsin inhibitor (equimolar amount to trypsin). Papain digestions with 1% (w/w) papain and 0.01 M cysteine, pepsin digestion with 1% (w/w) pepsin were performed as reported previously [7].
3.7. Polyacrylamide gel electrophoresis Acrylamide gel electrophoresis in a gradient from 5 to 30% acrylamide was performed as described by Margolis and Kenrick [9], except that a concave instead of a linear gradient was used. Polyacrylamide gel electrophoresis in SDS was performed as described by Laemmli [10] with the modifications previously reported [7,11 ]. 4. Results Fig. 1 shows the pattern obtained by immunoelectrophoresis analysis of monoclonal proteins belonging to the different IgG subclasses. Monoclonal IgG2b and IgG2c digested with 2% trypsin showed precipitation
IgG I
native
anti- rat serum 2%trypsin 4h anti- ~ 1 18h ............. i ' - , ,
2% trypsln 4h cys teine 0,01NIl
an It"mra t
serum
a n t i - 1S1
I gl~ 2 a native serum 2%trypsin a
serum
3.5. Purification of papain Fab and Fc fragments Papain Fab and Fc from monoclonal IgG 1 (IR 595), IgG2a (IR 418, IR 530) and IgG2a from normal rat serum were prepared by fractionation of a 2-h papain digest on an Ultrogel AcA44 (LKB, Sweden) column equilibrated in 0.01 M sodium phosphate, 0.15 M NaC1, pH 7.4. The Fab and Fc were further purified by chromatography on DEAE-ceUulose (DE 32 Whatman) and sequential elution with 0.01 M sodium phosphate, pH 7.8, then with sodium phosphate, pH 6.8, buffers of the following molarities: 0.01 M, 0.05 M and 0.2 M. Purification of Fab from monoclonal IgG2c (IR 304) was performed as for IgG 1 and lgG2a except that fractionation on Ultrogel AcA44 was omitted. 3.6. Immunoelectrophoresis Immunoelectrophoresis was carried out in 1% agarose with sodium barbital buffer 0.078 M, pH 8.2. 94
2%trypsin cysteineOj,,m .................. . . . . .
gn~z- u ~ a
[ I J l ~ 2 b native anti- rat serum 2%trypsln
2%trypsin 4h cystetne U I M Ig(~ 2c
anti- ]S2b
native
serum 2%trypsin 4 h ¢
18h anti- rat serum 2% trypsin 4 h
(ysteine 0.01M
~2c
Fig.1. Immunoelectrophoresisof monoclonal rat IgG native and digested with 2% (w/w) trypsin with or without 0.01 M cysteine. Anti--Y2cis also anti-idiotypic for the IgG2c protein (IR 304).
A
lines of Fab and Fc mobilities together with, in 4-h digests, a precipitation line of mobility similar to undigested protein. The addition of cysteine did not seem to significantly alter the immunoelectrophoretic pattern of these two subclasses. IgG2c from normal rat serum gave essentially the same results as monoclonal IgG2c (data not shown). In contrast to IgG2b and IgG2c, no Fab and Fc lines were observed in the digests of IgG 1 and IgG2a. For monoclonal IgG1, the
II
,~
Fig. 2. Electrophoresis in an acrylamide gradient (5-30%) of monoclonalIgG. 1, IgG1 (IR 27); 2, IgG2a (IR 418); 3, IgG2b (IR 863); 4, IgG2c (IR 304). A: native proteins; B: proteins digested for 4 h with 2% (w/w) trypsin.
A
a
120
b
c
d
~i ¸ i
~
~i~
i !i
•
. _. 120
!
90
13
13 O
50
13
II H~
1
Fig. 3. PAGE-SDS (gradient 5-25% acrylamide) of tryptic digests of rat lgG (digestion 4 h with 2% (w/w) trypsin). A: unreduced digests. B: digests reduced by 2-mercaptoethanol (5% final concentration). (a) IgG1 IR 595 ; (b) IgG1 IR 656; (c) IgG2a IR 530; (d) IgG2a IR 418; (e) IgG2a IR 33; (f) IgG2b IR 863; (g) IgG2c IR 304; (h) IgG2c IR 558; (i) IgG2c IR 759; (j) IgG2a from normal rat serum; (k) IgG2c from normal rat serum.
95
precipitation line, different from the one of undigested protein, has a cathodic part not reacting with an antiserum specific for IgG~ Fc. The time of incubation and the addition of 0.01 M cysteine did not seem to modify the immunoelectrophoretic pattern. The main compound observed in the 4-h digests of monoclonal IgG2a had the same mobility as the undigested protein. After 18 h of incubation, or with addition of 0.01 M cysteine, a precipitation line of mobility similar to that for pepsin Fc was observed. In comparison to monoclonal IgG2a, IgG2a from normal rat serum gave minor Fab and Fc lines in addition to undigested protein. Electrophoresis in a gradient of acrylamide (Fig. 2) of 4-h tryptic digests with 2% trypsin showed that monoclonal IgG2a appeared almost completely resisTRYPSIN 2%
PAPAIN 1%
tant to trypsin. For IgG1, about half of the preparation was found cleaved in fragments of high molecular weight (about 100,000 daltons). In the case of lgG2b and IgG2c most of the protein was split into components of tool. wt. 50,000. Polyacrylamide gel electrophoresis in SDS of different monoclonal IgG, together with IgG2a and IgG2c isolated from normal rat serum, after trypsin cleavage (4 h with 2% trypsin) is shown in Fig. 3. Monoclonal IgG 1 and IgG2a were found completely split into two main fragments: tool. wt. 120,000 and 13,000. In the case of IgG2a isolated from normal rat serum, an additional component of 52,000 daltons was also observed. Monoclonal lgG2b digests showed two components of 52,000 and 70,000 daltons. Monoclonal IgG2c and IgG2c from normal rat serum were split
TRY~IN 4.
CONTROLS
PAPAIN
with cystein
Fab
,q,
2% T RYPSIN
Fc
Fab
Fc
120
4il 30 13
13
27
~
27
~
13
B H chain 52 30 L chaln 23 13
27 23
7 3 13
23 l i
27 23 13 8
Fig. 4. PAGE-SDS (5 -25% acrylamide) of monoclonal IgG2a IR 418 digested with trypsin (2%, 4 h), papain (1%, 2 h with 0.01 M cysteine), trypsin plus papain, and of Fab and Fc from IR 418 digested with trypsin (2%, 4 h). A: unreduced digests; B: digests reduced by 2-mercaptoethanol (5% final concentration).
96
into a main component: mol. wt. 50,000. Some minor components of tool. wt. 100,000, 66,000 and 11,000 (the latter only noticed in IR 759 digests) were also observed. After reduction of the cleavage products by 2-mercaptoethanol (Fig. 3B) fragments of tool. wt. 38,000 (IgG1) or 36,000 (IgG2a) and of mol. wt. 13,000 were observed together with the light chain in monoclonal IgG 1 and IgG2a digests whereas the intact heavy chain was absent. For IgG2b a major component ofmol, wt. 27,000 and a minor of tool. wt. 36,000 were found associated with the light chain. IgG2c digests showed protein bands of mol. wt. 25,000 and 20,000 together with the light chain (and also low amounts of intact heavy chain). Taken together these results suggested that IgG2b and IgG2c were split by trypsin mainly into Fab and Fc, whereas a different type of cleavage had occurred for IgG 1 and IgG2a. For these two subclasses, right chains appeared undigested whereas the heavy chain was sprit into two fragments, tool. wt. 13,000 and 36,000 or 38,000. The results of PAGE-SDS analysis suggested that the large heavy chain fragments (mol. wt. 36,00038,000) were linked to the light chains to constitute the component of tool. wt. 120,000. In order to determine whether the cleavage of the heavy chain of rat IgG 1 and IgG2a was in the Fd or the Fc part of the IgG molecule, a comparison with the products of papain cleavage was undertaken. Papain releases from lgG 1 and IgG2a a non-covalent Fc (mol. wt. 27,000 for IgG2a ) and Fab fragments (mol. wt. 46,000-48,000) [7]. Different monoclonal IgG 1 and IgG2a were sprit successively with trypsin and papain (or the reverse) and the resulting fragments were compared to purified Fab and Fc by PAGE-SDS analysis. Also the digestion by trypsin of some Fab and Fc from monoclonal IgG 1 and IgG2a was investigated. The results shown for a monoclonal IgG2a (IR 418) (Fig. 4) indicated clearly that Fab was cleaved by trypsin into two components: mol. wt. 30,000 and 13,000 whereas Fc appeared undigested. After reduction by 2-mercaptoethanol, the component of mol. wt. 30,000 was found to be composed of a light chain (molz wt. 23,000)and of a component of tool. wt. 8,000. The same results were observed for the other monoclonal IgG 1 and IgG2a. In contrast to the Fab of monoclonal IgG 1 and IgG2a , Fab ofmonoclonal IgG2c (IR 304) was found insensitive to trypsin cleavage. Furthermore, pepsin plus trypsin cleavage
of monoclonal IgG 1 and IgG2a showed that F(ab)'2 was also completely cleaved by trypsin. These results showed that trypsin cleavage of rat IgG 1 and IgG2a occurs in the Fd part of the molecule with the release of a fragment ofmol, wt. 13,000 related to the N-terminal part of the heavy chain.
5. Discussion
Results of these studies showed that rat IgG are differentially affected by tryptic cleavage and that a relation to the IgG subclass can be defined. IgG2b and IgG2c are split mainly into Fab- and Fc-like fragments although minor components are also released. In the case of IgG2c, a large product (tool. wt. 100,000) probably an F(ab)2-1ike fragment is observed after a short time of cleavage. In IgG2b digests, the presence of a heavy chain component of tool. wt. 36,000 (i.e. like in IgG 1 and IgG2a digests but without the associated component of tool. wt. 13,000) suggests that a type of cleavage other than production of Fab and Fc has also occurred, but the nature of this cleavage has not been defined. For monoclonal IgG 1 and IgG2a, the results indicate that short time (4 h) tryptic cleavage sprit the heavy chain in the Fd with the release of a fragment corresponding to the N-terminal part of the heavy chain. The size of this fragment (tool. wt. 13,000) suggests that it probably represents most of the variable domain of the heavy chain. Such a cleavage has not been described for IgG of other species except occasionally for a monoclonal human IgG2 [12] and a monoclonal human IgG3 [13]. Production of a VH fragment has also been reported from papain digestion of a monoclonal human IgG 3 [14]. In all these reported cases the site of cleavage is situated in the third hypervariable region and therefore this explains that it occurs only for very peculiar sequences of this hypervariable zone. On the contrary to these previous reports cleavage of rat IgG 1 and IgG2a appears related to the subclass. In effect, it was observed for several different monoclonal IgG 1 and IgG2a and also for IgG2a from normal rat serum. However, in the case of this latter, an additional component of mol. wt. 52,000 was observed and the presence of minor Fab and Fc lines were demonstrated by immunoelectrophoresis. It was also observed that Fab of normal rat IgG2a was not entirely sprit by tripsin. This suggests 97
that a small part of the population of rat IgG2a is split into Fab and Fc and unsplit in the Fd part. It should also be noticed that long-time tryptic cleavage (18 h) or digestion in the presence of cysteine releases from monoclonal IgG2a a fragment probably related to the tFc described by Medgyesi [1]. Experiments are now in progress to determine the sequence around the cleavage site for IgG 1 and IgG2a. Nevertheless, at present, results of our studies indicate that it is easy to obtain a fragment related to the VH domain by tryptic cleavage of monoclonal rat IgG 1 and IgG2a whereas such a fragment cannot be obtained from IgG2b and IgG2c proteins.
Acknowledgements The helpful technical assistance of A. M. DeMv~re, J. Herno and M. T. Picqu6 is gratefully acknowledged. Studies have been supported by LA 268.04 CNRS (G. B.) and by CRL no. 80.3028 INSERM.
98
References [ 1 ] Medgyesi, G. A., Jakab, M., Cs6csinagy, M. and Gergely, J. (1971) Acta Biochim. Biophys. Acad. Sci. Hung. 6, 405-414. [2] Gorini, G., Medgyesi, G. A. and Doria, G. (1969) J. Immunol. 103, 1132-1142. [3] Nezlin, R. S., Krilov, M. Y. and Rokhlin, O. V. (1973) lmmunochemistry 10, 651-652. [4] Bazin, H., Beckers, A., Deckers, C. and Heremans, J. F. (1972) Eur. J. Cancer 10,568-580. [5] Bazin, H., Beckers, A., Deckers, C. and Moriamd, C. (1973) J. Natl. Cancer Inst. 51, 1359-1361. [6] Bazin, H., Beckers, A. and Querinjean, P. (1974) Eur. J. Immunol. 4, 44-48. [7[ Rousseaux, J., Biserte, G. and Bazin, H. (1980) Mol. Immunol. 17,469-482. [8] Rousseaux, J., Bazin, H. and Biserte, G. (1981) Mol. Immunol. (in press). [9] Margolis, J. and Kenrick, K. G. (1968) Anal. Biochem. 25,347-362. [ 10[ Laemmli, U. K. (1970) Nature (London) 227,680-685. [11] Kerckaert, J. P. (1978) Anal. Biochem. 84,354 360. [12] Laschinger, C. A. and ConneU, G. E. (1978) Immunochemistry 15,119-123. [13] Parr, D. H. and Hofmann, T. (1980) Mol. Immunol. 17, 1-7. [14] Dammaco, F., Franklin, E. C. and Frangione, B. (1972) J. Immunol. 109,565-569.
Elsevier/North-HollandBiomedicalPress
T R Y P T I C C L E A V A G E O F R A T lgG: A C O M P A R A T I V E S T U D Y B E T W E E N S U B C L A S S E S J. ROUSSEAUX, R. ROUSSEAUX-PREVOST, H. BAZIN* and G. BISERTE Institut de Reeherches sur le Cancer, INSERM U 124, BP 311, 59020 Lille Cddex, France and *Experimental Immunology Unit, Faculty of Medicine, University of Louvain, Clos-Chapelle-aux-Champs, 30-1200 Brussels, Belgium
(Received 15 September 1980) (Revised version received 13 February 1981) (Accepted 13 February 1981)
1. Summary Monoclonal rat IgG belonging to the 4 rat IgG subclasses, and some IgG subclasses isolated from normal rat serum were subjected to enzymatic degradation with trypsin. Differences in the products of tryptic digestion were observed according to the IgG subclass. IgG2b and IgG2c were degraded mainly into Fab and Fc fragments. IgG 1 and IgG2a appeared resistant to such cleavage. However, tryptic digestion of the latter two produced two fragments separated only in dissociating media. Results of the studies suggest that one of the fragments (mol. wt. 13,000) probably consists of most of the variable domain of the 3' heavy chain, while the second (mol. wt. 120,000) consists of the IgG deleted of the VH regions.
sin than IgG2a and IgG2b: these latter two are split into Fab- and Fc-like fragments that are not found in the digests of IgG 1. In the rat, Nezlin et al. [3] have demonstrated a different susceptibility to trypsin of two IgG subclasses: IgG2a and IgG2b. IgG2a is resistant to the enzyme and left undigested whereas IgG2b is degraded into Fab and Fc fragments. The availability of the monoclonal IgG secreted by the LOU rat immunocytomas [4,5] led us to extend the study to a comparison of the 4 rat IgG subclasses: IgG1, IgG2a, IgG2b and IgG2c [6]. Our results showed that IgG2b and IgG2c are susceptible to trypsin and split mainly into Fab. and Fc-like fragments. On the contrary, IgG 1 and IgG2a appeared mainly unsplit when studied in non-dissociating conditions, but were separated in dissociating media into two components, one of which is probably related to most of the VH domain and the other consists in the IgG depleted of VH.
2. Introduction Enzymatic hydrolysis has been used for several years as a means for immunochemical analysis of the immunoglobulins, in particular for the characterization of the subclasses. It has been shown that IgG subclasses differ in their sensitivity to the proteolytic enzymes, especially papain and pepsin. The action of trypsin has also been studied but less extensively. Human IgG1, IgG 3 and IgG4 are susceptible to trypsin cleavage, which produces Fab- and Fc-like fragments: tFab and tFc [ 1]. On the other hand, IgG 2 is resistant to trypsin and fragmentation occurs only after long times of digestion. In the mouse, studies by Gorini et al. [2] have shown that IgG 1 is more resistant to tryp-
3. Materials and methods 3.1. l m m u n o c y t o m a s Immunocytomas producing monoclonal immunoglobulins were maintained in our laboratory as described previously [4]. 3.2. Purification o f rat immunoglobulins
Normal rat serum was obtained from randomly bred Wistar rats. Monoclonal rat IgG and IgG2a from normal rat serum were isolated as described previously [7]. IgG2c from normal rat serum was purified by chromatog-
0165-2478/81/0000-0000/$2.50 © Elsevier/North-HollandBiomedicalPress
93
raphy of a precipitate at 40% ammonium sulphate saturation on a protein A-Sepharose (Pharmacia, Uppsala, Sweden) column. Elution was performed sequentially with 0.01 M sodium phosphate, 0.15 M NaC1 pH 8.0, 0.1 M sodium citrate, pH 6.0 and pH 3.0. Fraction eluted at pH 3.0 contains only IgG2c and represents about 90% of IgG2c from normal rat serum [81. 3.3. Antisera Polyvalent rabbit antisera to normal rat serum proreins were purchased from Sebia. Antisera specific for the heavy chain of rat IgG were prepared as described previously [6].
3.4. Enzyma tic digestions TPCK-trypsin was purchased from Worthington Biochemicals. Mercuripapain (25 mg/ml) and twice crystallized pepsin were from Sigma Chemicals. Tryptic digestions were performed as follows: rat IgG (10 mg/ml) in 0.1 M Tris • HC1 buffer, pH 7.8, containing 0.02 M CaC12, were digested for 4 h or 18 h with an enzyme:protein ratio of 2% (w/w) with or without the addition of 0.01 M cysteine. The digestion was terminated by addition of soybean trypsin inhibitor (equimolar amount to trypsin). Papain digestions with 1% (w/w) papain and 0.01 M cysteine, pepsin digestion with 1% (w/w) pepsin were performed as reported previously [7].
3.7. Polyacrylamide gel electrophoresis Acrylamide gel electrophoresis in a gradient from 5 to 30% acrylamide was performed as described by Margolis and Kenrick [9], except that a concave instead of a linear gradient was used. Polyacrylamide gel electrophoresis in SDS was performed as described by Laemmli [10] with the modifications previously reported [7,11 ]. 4. Results Fig. 1 shows the pattern obtained by immunoelectrophoresis analysis of monoclonal proteins belonging to the different IgG subclasses. Monoclonal IgG2b and IgG2c digested with 2% trypsin showed precipitation
IgG I
native
anti- rat serum 2%trypsin 4h anti- ~ 1 18h ............. i ' - , ,
2% trypsln 4h cys teine 0,01NIl
an It"mra t
serum
a n t i - 1S1
I gl~ 2 a native serum 2%trypsin a
serum
3.5. Purification of papain Fab and Fc fragments Papain Fab and Fc from monoclonal IgG 1 (IR 595), IgG2a (IR 418, IR 530) and IgG2a from normal rat serum were prepared by fractionation of a 2-h papain digest on an Ultrogel AcA44 (LKB, Sweden) column equilibrated in 0.01 M sodium phosphate, 0.15 M NaC1, pH 7.4. The Fab and Fc were further purified by chromatography on DEAE-ceUulose (DE 32 Whatman) and sequential elution with 0.01 M sodium phosphate, pH 7.8, then with sodium phosphate, pH 6.8, buffers of the following molarities: 0.01 M, 0.05 M and 0.2 M. Purification of Fab from monoclonal IgG2c (IR 304) was performed as for IgG 1 and lgG2a except that fractionation on Ultrogel AcA44 was omitted. 3.6. Immunoelectrophoresis Immunoelectrophoresis was carried out in 1% agarose with sodium barbital buffer 0.078 M, pH 8.2. 94
2%trypsin cysteineOj,,m .................. . . . . .
gn~z- u ~ a
[ I J l ~ 2 b native anti- rat serum 2%trypsln
2%trypsin 4h cystetne U I M Ig(~ 2c
anti- ]S2b
native
serum 2%trypsin 4 h ¢
18h anti- rat serum 2% trypsin 4 h
(ysteine 0.01M
~2c
Fig.1. Immunoelectrophoresisof monoclonal rat IgG native and digested with 2% (w/w) trypsin with or without 0.01 M cysteine. Anti--Y2cis also anti-idiotypic for the IgG2c protein (IR 304).
A
lines of Fab and Fc mobilities together with, in 4-h digests, a precipitation line of mobility similar to undigested protein. The addition of cysteine did not seem to significantly alter the immunoelectrophoretic pattern of these two subclasses. IgG2c from normal rat serum gave essentially the same results as monoclonal IgG2c (data not shown). In contrast to IgG2b and IgG2c, no Fab and Fc lines were observed in the digests of IgG 1 and IgG2a. For monoclonal IgG1, the
II
,~
Fig. 2. Electrophoresis in an acrylamide gradient (5-30%) of monoclonalIgG. 1, IgG1 (IR 27); 2, IgG2a (IR 418); 3, IgG2b (IR 863); 4, IgG2c (IR 304). A: native proteins; B: proteins digested for 4 h with 2% (w/w) trypsin.
A
a
120
b
c
d
~i ¸ i
~
~i~
i !i
•
. _. 120
!
90
13
13 O
50
13
II H~
1
Fig. 3. PAGE-SDS (gradient 5-25% acrylamide) of tryptic digests of rat lgG (digestion 4 h with 2% (w/w) trypsin). A: unreduced digests. B: digests reduced by 2-mercaptoethanol (5% final concentration). (a) IgG1 IR 595 ; (b) IgG1 IR 656; (c) IgG2a IR 530; (d) IgG2a IR 418; (e) IgG2a IR 33; (f) IgG2b IR 863; (g) IgG2c IR 304; (h) IgG2c IR 558; (i) IgG2c IR 759; (j) IgG2a from normal rat serum; (k) IgG2c from normal rat serum.
95
precipitation line, different from the one of undigested protein, has a cathodic part not reacting with an antiserum specific for IgG~ Fc. The time of incubation and the addition of 0.01 M cysteine did not seem to modify the immunoelectrophoretic pattern. The main compound observed in the 4-h digests of monoclonal IgG2a had the same mobility as the undigested protein. After 18 h of incubation, or with addition of 0.01 M cysteine, a precipitation line of mobility similar to that for pepsin Fc was observed. In comparison to monoclonal IgG2a, IgG2a from normal rat serum gave minor Fab and Fc lines in addition to undigested protein. Electrophoresis in a gradient of acrylamide (Fig. 2) of 4-h tryptic digests with 2% trypsin showed that monoclonal IgG2a appeared almost completely resisTRYPSIN 2%
PAPAIN 1%
tant to trypsin. For IgG1, about half of the preparation was found cleaved in fragments of high molecular weight (about 100,000 daltons). In the case of lgG2b and IgG2c most of the protein was split into components of tool. wt. 50,000. Polyacrylamide gel electrophoresis in SDS of different monoclonal IgG, together with IgG2a and IgG2c isolated from normal rat serum, after trypsin cleavage (4 h with 2% trypsin) is shown in Fig. 3. Monoclonal IgG 1 and IgG2a were found completely split into two main fragments: tool. wt. 120,000 and 13,000. In the case of IgG2a isolated from normal rat serum, an additional component of 52,000 daltons was also observed. Monoclonal lgG2b digests showed two components of 52,000 and 70,000 daltons. Monoclonal IgG2c and IgG2c from normal rat serum were split
TRY~IN 4.
CONTROLS
PAPAIN
with cystein
Fab
,q,
2% T RYPSIN
Fc
Fab
Fc
120
4il 30 13
13
27
~
27
~
13
B H chain 52 30 L chaln 23 13
27 23
7 3 13
23 l i
27 23 13 8
Fig. 4. PAGE-SDS (5 -25% acrylamide) of monoclonal IgG2a IR 418 digested with trypsin (2%, 4 h), papain (1%, 2 h with 0.01 M cysteine), trypsin plus papain, and of Fab and Fc from IR 418 digested with trypsin (2%, 4 h). A: unreduced digests; B: digests reduced by 2-mercaptoethanol (5% final concentration).
96
into a main component: mol. wt. 50,000. Some minor components of tool. wt. 100,000, 66,000 and 11,000 (the latter only noticed in IR 759 digests) were also observed. After reduction of the cleavage products by 2-mercaptoethanol (Fig. 3B) fragments of tool. wt. 38,000 (IgG1) or 36,000 (IgG2a) and of mol. wt. 13,000 were observed together with the light chain in monoclonal IgG 1 and IgG2a digests whereas the intact heavy chain was absent. For IgG2b a major component ofmol, wt. 27,000 and a minor of tool. wt. 36,000 were found associated with the light chain. IgG2c digests showed protein bands of mol. wt. 25,000 and 20,000 together with the light chain (and also low amounts of intact heavy chain). Taken together these results suggested that IgG2b and IgG2c were split by trypsin mainly into Fab and Fc, whereas a different type of cleavage had occurred for IgG 1 and IgG2a. For these two subclasses, right chains appeared undigested whereas the heavy chain was sprit into two fragments, tool. wt. 13,000 and 36,000 or 38,000. The results of PAGE-SDS analysis suggested that the large heavy chain fragments (mol. wt. 36,00038,000) were linked to the light chains to constitute the component of tool. wt. 120,000. In order to determine whether the cleavage of the heavy chain of rat IgG 1 and IgG2a was in the Fd or the Fc part of the IgG molecule, a comparison with the products of papain cleavage was undertaken. Papain releases from lgG 1 and IgG2a a non-covalent Fc (mol. wt. 27,000 for IgG2a ) and Fab fragments (mol. wt. 46,000-48,000) [7]. Different monoclonal IgG 1 and IgG2a were sprit successively with trypsin and papain (or the reverse) and the resulting fragments were compared to purified Fab and Fc by PAGE-SDS analysis. Also the digestion by trypsin of some Fab and Fc from monoclonal IgG 1 and IgG2a was investigated. The results shown for a monoclonal IgG2a (IR 418) (Fig. 4) indicated clearly that Fab was cleaved by trypsin into two components: mol. wt. 30,000 and 13,000 whereas Fc appeared undigested. After reduction by 2-mercaptoethanol, the component of mol. wt. 30,000 was found to be composed of a light chain (molz wt. 23,000)and of a component of tool. wt. 8,000. The same results were observed for the other monoclonal IgG 1 and IgG2a. In contrast to the Fab of monoclonal IgG 1 and IgG2a , Fab ofmonoclonal IgG2c (IR 304) was found insensitive to trypsin cleavage. Furthermore, pepsin plus trypsin cleavage
of monoclonal IgG 1 and IgG2a showed that F(ab)'2 was also completely cleaved by trypsin. These results showed that trypsin cleavage of rat IgG 1 and IgG2a occurs in the Fd part of the molecule with the release of a fragment ofmol, wt. 13,000 related to the N-terminal part of the heavy chain.
5. Discussion
Results of these studies showed that rat IgG are differentially affected by tryptic cleavage and that a relation to the IgG subclass can be defined. IgG2b and IgG2c are split mainly into Fab- and Fc-like fragments although minor components are also released. In the case of IgG2c, a large product (tool. wt. 100,000) probably an F(ab)2-1ike fragment is observed after a short time of cleavage. In IgG2b digests, the presence of a heavy chain component of tool. wt. 36,000 (i.e. like in IgG 1 and IgG2a digests but without the associated component of tool. wt. 13,000) suggests that a type of cleavage other than production of Fab and Fc has also occurred, but the nature of this cleavage has not been defined. For monoclonal IgG 1 and IgG2a, the results indicate that short time (4 h) tryptic cleavage sprit the heavy chain in the Fd with the release of a fragment corresponding to the N-terminal part of the heavy chain. The size of this fragment (tool. wt. 13,000) suggests that it probably represents most of the variable domain of the heavy chain. Such a cleavage has not been described for IgG of other species except occasionally for a monoclonal human IgG2 [12] and a monoclonal human IgG3 [13]. Production of a VH fragment has also been reported from papain digestion of a monoclonal human IgG 3 [14]. In all these reported cases the site of cleavage is situated in the third hypervariable region and therefore this explains that it occurs only for very peculiar sequences of this hypervariable zone. On the contrary to these previous reports cleavage of rat IgG 1 and IgG2a appears related to the subclass. In effect, it was observed for several different monoclonal IgG 1 and IgG2a and also for IgG2a from normal rat serum. However, in the case of this latter, an additional component of mol. wt. 52,000 was observed and the presence of minor Fab and Fc lines were demonstrated by immunoelectrophoresis. It was also observed that Fab of normal rat IgG2a was not entirely sprit by tripsin. This suggests 97
that a small part of the population of rat IgG2a is split into Fab and Fc and unsplit in the Fd part. It should also be noticed that long-time tryptic cleavage (18 h) or digestion in the presence of cysteine releases from monoclonal IgG2a a fragment probably related to the tFc described by Medgyesi [1]. Experiments are now in progress to determine the sequence around the cleavage site for IgG 1 and IgG2a. Nevertheless, at present, results of our studies indicate that it is easy to obtain a fragment related to the VH domain by tryptic cleavage of monoclonal rat IgG 1 and IgG2a whereas such a fragment cannot be obtained from IgG2b and IgG2c proteins.
Acknowledgements The helpful technical assistance of A. M. DeMv~re, J. Herno and M. T. Picqu6 is gratefully acknowledged. Studies have been supported by LA 268.04 CNRS (G. B.) and by CRL no. 80.3028 INSERM.
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