Antigenic diversity of IgA receptors in Streptococcus pyogenes

Antigenic diversity of IgA receptors in Streptococcus pyogenes

FEMS Immunology and Medical Microbiology 7 (1993) 47-54 © 1993 Federation of European Microbiological Societies 0920-8534/93/$06.00 Published by Elsev...

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FEMS Immunology and Medical Microbiology 7 (1993) 47-54 © 1993 Federation of European Microbiological Societies 0920-8534/93/$06.00 Published by Elsevier

47

FEMSIM 00322

Antigenic diversity of IgA receptors in Streptococcus pyogenes Larissa A. Burova

a

and Claes Schal6n b

Institute of Experimental Medicine, Academy of the Medical Sciences, St. Petersburg, Russia, and 6 Department of Medical Microbiology, University of Lund, Lund, Sweden (Received 8 September 1992; revision received 7 January 1993; accepted 20 January 1993)

Abstract." In a previous study, group A and group B streptococcal lgA receptors were shown to differ serologically, in agreement with their known structural unrelatedness. The present study was undertaken to serologically compare the lgA binding epitopes of group A streptococcal strains representing various serotypes by the use of antisera to this species. It was found that blocking antibodies occurred in antisera to IgA binding but not to non-binding strains and that binding of IgA to a streptococcal strain was generally blocked by antiserum to the homologous type. However, cross-testing of a panel of 11 IgA binding strains, representing various M and T serotypes, with 10 different antisera to group A streptococci, demonstrated that IgA receptors were inhibited to a highly variable degree and that inhibition patterns were unique for each type. Comparing solubilized IgA receptors of various strains in immunoblot experiments, a variation in the molecular mass, between approximately 35 and 45 kDa, emerged. The IgA binding epitopes, analogous to protective sites of streptococcal M-protein, thus exhibited hypervariability which may suggest that IgA binding also plays a key role for evading host immune defence mechanisms.

Key words: Streptococcus pyogenes; IgA; Fc-receptor

Introduction

Bacterial proteins interacting with immunoglobulins are increasingly recognized [1]. In particular, surface Fc-binding proteins (Fc-receptors) specific for IgG occur in some pathogens of major importance, such as Staphylococcus aureus (protein A), group A streptococci and groups C / G streptococci (protein G). As currently known, receptors specific for IgA are limited to

Correspondence to." C. Schal~n, Department of Medical Microbiology, University of Lurid, S61vegatan 23, S-223 62 Lund, Sweden.

two species, groups A and B streptococci [2,3]. The biologic role of these Fc-receptors is less well known; however, blocking of opsonophagocytosis and triggering of anti-IgG production by IgG Fc-receptors may represent mechanisms of relevance for pathogenicity [4-6]. It is believed that IgA Fc-receptors, by their binding of secretory antibodies, may have a role in the mucosal establishment of the organisms. In contrast to protein A and protein G, group A streptococcal IgG Fc-receptors are not uniform in structure or in binding of different IgG molecules [7,8]. Recently, these IgG Fc-receptors were thus subdivided into several categories by their patterns of binding human IgG subclasses

48 [9]. Furthermore, gene sequencing has revealed IgG Fc-receptors of two types of group A streptococci to be structurally unrelated [10,11]. Previously group A streptococcal binding of IgG and IgA was demonstrated to occur by distinct and separable sites [12,13]. Recent findings by gene cloning, however, have suggested the two types of Fc-receptor as often molecularly linked [14]. It is also becoming evident that the Fc-receptor molecules are closely related to streptococcal M protein and that genetic recombination may account for a functional differentiation among these virulence factors [15,16]. Though IgA binding occurs exclusively in group A and group B streptococci, the IgA Fc-receptors of the two species are structurally unrelated, and the cloned and purified receptors were found not to cross-react immunologically [17]. In accordance, the IgA binding epitopes of the two species were also demonstrated to be serologically distinct [18]. This was analogous to protein A, protein G and group A streptococcal IgG Fc-receptor respectively, which, in spite of binding the same site of IgG [19], were found structurally and antigenically distinct [20]. With reference to group A streptococci, gene sequencing of IgA Fc-receptors of two types, M4 and M60, showed these to be closely similar except for the N-terminal, IgA-binding region [21]. On the other hand, the overall sequence of another IgA Fc-receptor, of type M2, was found to be clearly distinct from these and more closely related to M-protein of another type [22]. As with streptococcal M-protein, exhibiting type-specific, protective determinants [23], it seemed possible that immunglobulin receptors might be further differentiated in their critical, Fc-binding epitopes due to selection by host immunity. This concept was tested in the present work by the use of IgA receptor blocking antibodies in rabbit antisera to group A streptococci.

Materials and M e t h o d s

Bacterial strains A collection of 43 reference strains of group A streptococci (Streptococcus pyogenes), represent-

ing different M-types, had been earlier obtained from the W H O Streptococcal Reference Laboratory, Prague. The type M49 strain no. 256 was from a collection of nephritogenic strains of Dr. J. Sramek, Prague. The M2 strain no. 245 was from Prof. S. Holm, Ume~. In addition, five IgA binding strains of various T-types, selected from a recent study [18], were included. The strains were kept frozen in calf serum at -80°C.

Chemicals Monoclonal IgA1, isolated from serum of a patient with myeloma, was kindly provided by Prof. A. Grubb, Lund. Labelling with 125I w a s performed using Iodobeads (Pierce Chemicals Co., Rockford, IL) according to the manufacturer's instructions. Rabbit antisera Rabbit type antisera to S. pyogenes types M2, M8, M25, M49 and M60 were kindly supplied by Dr. H. Havlickova, Prague. Rabbit antisera to S. pyogenes types M1, M4, M6, M15, M22 and M50 and S. agalactiae, type Ib, were raised by intravenous immunization according to Rotta et al. [24]. Binding of IgA to intact cells and blocking tests A radiobinding assay, earlier described [12], was used with minor modifications to determine the uptake of IgA on whole streptococci. Logarithmic or stationary phase bacterial cells, grown in Todd-Hewitt broth (Oxoid, Columbia, MD) between 4 and 18 h, as specified in Table 1, approximately 2 x 10 9 cfu in 200/zl of phosphate buffered saline (PBS) were mixed with 50 ~zl of 125I labelled IgA (0.2 /~g; specific activity 3 - 6 x 104 cpm) and incubated on a shaker at 20°C for 1 h. After washing with PBS containing 0.02% sodium azide and 0.1% Tween 20 (PBST) the radioactivity remaining in the pellet was counted in a gamma-counter (LKB Multigamma, Bromma, Sweden). Binding was expressed as per cent of added amount. The difference between duplicate determinations did not exceed 5% of the total count. For blocking of the IgA binding, 25 /zl of rabbit antiserum or various amounts of a strepto-

49 Table 1 Binding of IgA to streptococcal Fc-receptors as related to growth phase * Streptococcal strain

Binding of radiolabelled IgA (%) Early log-phase (4 h)

Late Stationary log-phase phase (8 h) (18 h)

Group A, type: M2 M4 M8 M22

27.1 35.9 36.1 56.0

9.4 32.0 12.5 46.0

13.1 30.2 10.7 42.8

M48 M49 M60

48.5 54.0 57.5

13.4 18.0 43.5

6.7 7.2 35.0

T2 T13 T25 T28

52.5 34.7 56.2 35.4

20.6 24.5 20.2 30.6

8.4 27.2 8.7 22.7

Group B, type: Ib

32.5

34.1

32.1

* Bacterial strains were grown in Todd-Hewitt broth at 37°C for indicated times. The cells were tested for binding of radiolabelled human IgA1 as described in Materials and Methods.

Tris buffer, pH 8.2. The cells were suspended in 5 ml Tris buffer, pH 8.2, containing 0.5 M sucrose. After adding 1.5 ml P A L the mixture was incubated at 37°C for 1 h and then benzamidine hydrochloride was added to a final concentration of 10 mM. Remaining cell debris was removed by centrifugation at 20000 x g and the supernatant, following sterile filtering through a 0.22/xm filter (Millipore, Bedford, MA), saved as P A L extract. Storage was at - 2 0 ° C . Solubilization of IgA-binding material was also attempted by mutanolysin extraction as follows: Log-phase bacteria were prepared as above by pretreatment with glycine and DL-threonine at 2 h and 1 h before harvesting, respectively. The cells were treated with mutanolysin (Sigma, St Louis, MO) 2 0 / z g / m l at 37°C for 3 h. Benzamidine hydrochloride was added and cells centrifuged as above. In some experiments, hot alkaline [26] or hot neutral extraction [8] was performed for solubilization. IgA Fc-receptor was released from group B streptococci, type Ib, by incubation of the cells at pH 11.0 as described by Lindahl et al. [27].

SDS-PAGE and immunoblot coccal extract was preincubated in duplicates with 200 /zl streptococcal suspension at 20°C for 1 h and the ceils were then washed twice with PBST. The cells were suspended in 200 /xl PBST and binding of radiolabelled IgA was then performed as above. The specific blocking was estimated by comparison with the IgA binding level obtained on preincubation with normal rabbit serum and was expressed as per cent reduced binding.

Solubilization of IgA binding proteins Phage associated lysin (PAL) was prepared from group C streptococci, strain 4540 (kindly provided by Dr. K.-H. Schmidt, Jena) according to Cohen et al. [25] with minor modifications. For solubilization of group A streptococcal IgA Fc-receptors by PAL, the bacterial strains were grown in 100 ml Todd Hewitt broth at 37°C for 4-18 h. Another 100 ml prewarmed broth was added at 3 h before and 5 g of glycine at 2 h before harvesting. The bacteria were deposited by centrifugation at 3000 x g and washed once in 20 mmol

S D S - P A G E was performed according to Laemmli [28]. A polyacrylamide concentration of 12% was used. Electroblotting onto Immobilon membranes (Millipore) was done in a TransBlot cell (Bio-Rad, McLean, VA). The membrane was blocked by PBS containing 1% gelatin and washed five times with PBST. For detection of immunoglobulin binding material the membrane was incubated with 1251 labelled monoclonal IgA for 3 h and then washed repeatedly in PBST. The membrane was applied onto a Kodak film and kept at - 7 0 ° C for 1-5 days until developed.

Results

Selection of S. pyogenes strains of various types for IgA binding capacity To compare IgA receptors derived from various types of S. pyogenes, a number of strains were selected and suitable growth conditions for binding of radiolabelled IgA were defined. The growth

50 100

t i m e r e q u i r e d for e x p r e s s i o n o f m a x i m a l I g A b i n d i n g activity o f t h e s t r a i n s w a s f o u n d to v a r y s i g n i f i c a n t l y b e t w e e n 4 a n d 18 h, as s h o w n in T a b l e 1. A n o p t i m a l t i m e o f i n c u b a t i o n , w h i c h a p p e a r e d c o n s i s t e n t f o r e a c h strain, w a s u s e d in t h e f o l l o w i n g , b o t h in b l o c k i n g e x p e r i m e n t s a n d for s o l u b i l i z a t i o n o f I g A F c - b i n d i n g m a t e r i a l . O u t o f 48 r e f e r e n c e s t r a i n s o f d i f f e r e n t M - o r T - t y p e s , s e v e n e x h i b i t e d b i n d i n g o f I g A at levels e x c e e d i n g 2 5 % a n d t h e r e f o r e w e r e c h o s e n for i n h i b i t i o n e x p e r i m e n t s . I n a d d i t i o n , f o u r I g A - b i n d i n g clinical i s o l a t e s o f d i f f e r e n t T - t y p e , s e l e c t e d f r o m a p r e v i o u s s t u d y [18] w e r e i n c l u d e d .

~3 c

•~ i

~

90 80

7o

N

60

g '~

50 40

.=-

30

"E

O

~

O

~ " 0 ~ 0

2o

0_

o

Anti-M5O O ~ o I

~

1

2



O ~ q p

~

I

5 10 Dilution of immune serum (x)

20

Inhibition of IgA binding to S. pyogenes strains by rabbit antisera to homologous and heterologous types

Fig. 1. Blocking of streptococcal IgA receptor by specific antibodies. Rabbit antisera against group A streptococci, types M22 and M50, were compared for inhibition of the binding of IgA to type M22, group A streptococci. Dilutions of the sera were preincubated with M22 streptococci at 37°C for 1 h and washed bacteria were then tested for binding of radiolabelled, human Igal as described in Materials and Methods.

I n i t i a l c o m p a r i s i o n s o f r a b b i t a n t i s e r a to t y p e s M 4 , M6, M 1 5 , M 2 2 a n d M 5 0 , p r o d u c e d in o u r institute, indicated that the blocking of IgA-binding d i f f e r e d m a r k e d l y b e t w e e n S. pyogenes s t r a i n s o f d i f f e r e n t t y p e s ( n o t shown). I n o r d e r to e x t e n d t h e s e o b s e r v a t i o n s , r a b b i t a n t i s e r a to a d d i t i o n a l types were obtained from the WHO Reference laboratory, Prague, which enabled the testing of

m o s t o f t h e s e l e c t e d s t r a i n s w i t h s e r a r a i s e d to the same type (though the M-type of the four T - t y p e d s t r a i n s was n o t d e t e r m i n e d ) . W e first e x a m i n e d w h e t h e r d i l u t i o n o f t h e s e r a u n d e r t e s t w o u l d still e n a b l e b l o c k i n g o f t h e I g A - b i n d i n g to b e d e m o n s t r a t e d . A s c a n b e s e e n

Table 2 Blocking of group A streptococcal IgA receptors by antisera to group A streptococci of various types

Streptococcus pyogenes type (strain) M2 M4 M8 M22 M48 M49 M60 T2 (10445) T13 (10487) T25 (10 494) T28 (10496) GBS. type Ib

Binding of IgA in presence of NRS

% inhibition of the binding of radiolabelled IgA to bacteria by rabbit antiserum to group A streptococci, type: M1

M2

M4

M6

M8

M15

M22

M49

M50

M60

GBS, Ib

26.3 35.9 20.9 * * 54.2 43.9 54.0 57.5 52.5 34.7 56.2 30.9 34.2

0 0 74.6 0 0 0 0 0 0 0 0 0

67.0 79.0 80.0 53.0 0 0 0 58.9 64.3 0 63.8 0

0 62.7 84.9 62.9 0 0 0 49.5 25.2 0 50.2 0

0 40.9 81.3 61.4 0 0 0 0 51.0 0 0 0

78.6 65.1 82.1 0 87.4 0 67.7 61.9 22.6 39.8 0 0

0 46.0 85.1 56.1 0 0 0 0 34.0 0 43.4 0

0 57.9 85.1 80.0 0 0 0 36.9 46.9 0 54.0 0

59.3 77.6 82.4 64.8 0 65.7 0 70.7 67.9 0 76.0 0

0 0 0 0 0 0 0 0 0 0 0 0

84.7 83.2 84.2 57.4 91.9 0 83.7 84.7 68.5 81.2 79.1 0

0 0 0 0 0 0 0 0 0 0 0 61.4

* The inhibition was calculated by comparison with the binding of radiolabelled IgA in the presence of NRS and expressed as per cent reduction in presence of immune serum. Means of duplicate determinations are given. * * The binding of IgA to M8 was reduced from 36.1% to 20.9% by NRS.

51

in Fig. 1, the use of undiluted serum seemed to allow optimal distinction between blocking and non-blocking sera. Sera were thus used undiluted in the following. Antiserum to non IgA-binding strain M50 did not inhibit the IgA binding to any of the strains. Similarly, as expected from our own data [18], antiserum to group B streptococci, type Ib was inhibitory for the binding of IgA to group B streptococci but not to any of the S. pyogenes strains. It was found that, for each of seven S. pyogenes strains, representing different M-types, binding of IgA was strongly inhibited by rabbit antiserum against the homologous type. Also, the binding of IgA to the T2 strain was inhibited by anti-M2 serum. However, binding of IgA to the T25 strain was not inhibited by anti-M25 serum, perhaps indicating that the strain was not of M-type 25 (not shown). When examining the inhibitory capacity of sera of heterologous M-types systematically, a more complex pattern was obtained (Table 2). Binding of IgA to the type T2 strain was thus inhibited by antisera to types M2, M4, M8, M22, M49 and M60 but not by those to types M1, M6 or M15. In contrast, the binding of IgA to M8 was strongly inhibited by all these antisera. Furthermore, the binding of IgA to M22, T13 and T28 was inhibited by each of the mentioned sera with the exception of anti-M1 and anti-M8. The binding to M4 was inhibited by these 9 antisera with the exception of anti-M1. In contrast, binding of IgA to M49 was inhibited only by anti-M49. The binding of IgA to T25, M48 and M60 was inhibited only by antisera to M8 and M60. The pattern of inhibition when comparing T2 and M2 was not identical since the binding to the former strain was inhibited by anti-M4 or anti-M22 serum.

Comparison of solubilized streptococcal IgA Fc-receptors of various types To compare the MW of IgA Fc-receptors of different types of group A streptococci, attempts to release these surface proteins were made. The capacity of crude extracts to inhibit binding of radiolabelled IgA to intact bacterial cells was used to detect solubilized receptors, as shown in Table 3. In the strains of types M2, M4, M8 and

Table 3 Inhibition of 1251 labelled IgA binding to Streptococcus pyogenes, type M22, by solubilized IgA Fc-receptors Group A streptococcal type

Volume tzl *

Per cent inhibition

100 100 100 25 25 50 25 25 50 50 50 100 50 50

6.2 10.5 12.7 65.2 31.4 67.9 51.0 11.1 33.0 26.9 45.6 13.1 33.5 27.8

100

24.5

Phage lysin extracts

M2 M4 M8 M22 M48 M48 M49 M60 M60 T2 T13 T25 T28 (10496) T28 (10499) Heat neutral extract * *

M22

* A volume of 25 p.l was used for screening all extracts for inhibitory capacity. In case of negative outcome, 50 and, finally, 100/zl volumes were tested. ** Heat neutral extracts of remaining S. pyogenes strains did not give inhibition.

T25 treatment of the cells by either PAL, mutanolysin, or heat neutral or alkaline extraction did not yield any functional IgA receptors. Also, for the strains tested of types M48, M49, M60, T2, T13, T28 (2 strains), there was no yield by mutanolysin, or heat neutral or alkaline extraction, whereas treatment with PAL resulted in solubilization of IgA receptors to various degrees as tested by inhibition of IgA-uptake. For M22, both heat neutral and PAL extracts gave inhibition of uptake. When tested by SDS-PAGE and immunoblot using radiolabelled IgA as probe, the solubilized IgA Fc-receptors of eight different types exhibited a range of MW between 35 and 45 kDa (Figure 2). Thus, the apparent MW of IgA Fc-receptors of M48 and M49 was clearly lower than those of M60, T2, T13 or T28. The type M22 IgA Fc-receptor exhibited a wide range of molecular species. As expected [27], a high MW of the IgA

Fig. 2. Comparison of IgA-binding proteins from various types of group A streptococci by Western blot analysis. PAL extracts were separated by SDS-PAGE and transferred to an Immobilon membrane by electroblotting. The membrane was exposed to radiolabelled human IgAl for 3 h at 22°C and then to an X-ray film for 5 days at - 80°C.

receptor of the group B streptococcal strain was noted.

type lb

Discussion

Groups A and B streptococci are unique among known bacteria in their binding of IgA Fc [2]. By utilizing blocking antibodies in rabbit antisera to streptococci we recently found that the non-immune binding of IgA to the two streptococcal species is mediated by serologically distinct epitopes [18]. In agreement, gene sequencing data have demonstrated these IgA receptors to be structurally unrelated, and the respective recombinant proteins did not cross-react immunologically [17]. Furthermore, among group B streptococci, binding of IgA is essentially limited to a less prevalent serotype, Ib, whereas in group A streptococci, IgA receptors are found in a high proportion of strains, though with various frequency among the serotypes [18,29,30]. It is of special interest that IgA as well as IgG Fc-receptors in S. pyogenes were recently found closely related to M-protein, a main virulence factor. The serologically diverse nature of M-protein might thus suggest a potential of variation also for these Fc-binding proteins.

The present results showed that, in 11 selected group A streptococcal strains of different type, binding of IgA to whole cells was generally blocked by rabbit antiserum to the homologous M-type. This confirmed our previous finding [181 that IgA receptor blocking antibodies are commonly evoked by immunization of rabbits. On the other hand, when comparing the blocking capacity of these and some other group A streptococcal antisera, using our panel of strains a complex pattern of inhibition occurred indicating a differentiation among these IgA binding proteins. For example, the binding of IgA to M49 was blocked by antiserum to the homologous type but unaffected by antisera raised to 7 heterologous types though with blocking capacity for at least one type. In contrast, binding of IgA to M4 was strongly blocked by the antisera to 7 heterologous types. Comparing the various antisera tested, antiM60 blocked the binding of IgA to all examined strains except a type M49 strain. In particular, the blocking of the IgA receptor of type M4 was of interest since the genes for IgA receptors in a type M4 and a type M60 strain were found similar except for the IgA-binding regions [211; also, anti-MS serum affected binding of IgA to M4 and M60 to a similar degree. However, the antisera to M4 and five other types were inhibitory for M4 but not M60, a finding demonstrating that M4 and M60 IgA receptors nevertheless differed conformationally. Binding of monoclonal, radiolabelled IgAl to whole streptococci was used for the detection of IgA receptor activity. It was previously demonstrated that the binding occurs by the Fc-part [12] and that human IgAl and IgA2 interact to a similar degree with the streptococcal receptors [17]. We found that antiserum to a non-IgA binding group A streptococcal strain, type M.50, did not affect the binding of IgA to any of the tested strains. Similarly, antiserum to group B streptococci type Ib was strongly inhibitory for the homologous strain but to none of the group A streptococcal strains. A remarkably fine specificity of the present blocking tests was thus evident, though whole cells and polyclonal antisera were employed. It was found, however, that anti-

53 serum to type M1, commonly binding IgG rather than IgA [18], was inhibitory for one of the IgA receptors, of a type M8 strain. This M8 strain was also exceptional by the finding that IgA binding was significantly reduced by normal rabbit serum, suggesting that IgA and IgG binding epitopes were sterically interfering; therefore, the marked inhibition obtained with M8 by some of the antisera might be caused by antibodies to the IgA receptor but also by normal rabbit IgG as well as putative antibodies blocking the IgG receptor. Normal rabbit IgA, though binding to streptococcal IgA receptor, did not appear inhibitory in our tests, possibly due to low levels in rabbit sera. Obviously, differences in density of the various IgA receptors on the intact ceils or in levels of IgA blocking antibodies in the various antisera could complicate interpretation of the present blocking tests. However, by the matching of a panel of strains and antisera, numerous positive and negative controls were provided at each testing occasion. Further tests with additional sera are required to see whether the strains of types M48, M60 and T25 might have similar or distinct IgA-binding epitopes. Our results revealed that at least 8 out of 11 group A streptococcal IgA binding strains tested exhibited unique inhibition patterns and thus antigenically distinct IgA binding epitopes. A hypervariation among IgA receptors of group A streptococci was thus indicated, a finding of interest due to their close relation to M proteins [15,16]. Whether the different IgA Fc-receptor variants might still bind to the same IgA site and with comparable affinity is of interest to study. It is also unclear at present whether these IgA receptors are serologically M-type specific, which might be suggested by the inhibition of IgA-binding by the respective, homologous antisera; however, immunizing and tested strains of the same M serotype were not identical in only two out of six combinations studied here. A variation in the IgA receptors of group A streptococci was also suggested by solubilization experiments, though the MW of seven out of eight successfully extracted receptors was within a relatively narrow range. For a long time, isolation of IgA receptors by various methods has met with limited success. The present results seem to

demonstrate that use of PAL rather than other means for extraction might be advantageous. However, the failure to recover functional IgA receptors from some of the strains showed that refinement of the present procedure is still desirable. In conclusion, an antigenic hypervariability of group A streptococcal IgA binding epitopes was demonstrated suggesting a crucial role for these Fc-receptors in pathogenicity. A similar conclusion may emerge from a structural and functional differentiation of IgG Fc receptors in this species now becoming evident [7,8]. Whether the clinical panorama of group A streptococcal manifestations may vary depending on the presence of the immunoglobulin Fc-receptors and immunity to these proteins is still unknown.

Acknowledgements The secretarial assistance of Ms. Gudrun Strand is gratefully acknowledged. The study was supported by grants from The Swedish Medical Research Council (P92-16X-09922-01A), The Royal Swedish Academy of Sciences (SwedishSoviet Scientific Cooperation), The Royal Physiographical Society of Lund, The Greta and Johan Kock Foundation, Trelleborg, The Alfred 0sterlund Foundation, Malm6, and the Medical Faculty, University of Lund.

References 1 Boyle, M.D.P. (1990) Bacterial Immunoglobulin-binding proteins, Vol. I. Academic Press, New York. 2 Christensen P. and Oxelius, V.-A. (1975) A reaction between some streptococci and IgA myeloma proteins. Acta Path. Microbiol, Scand. Sect. C 83, 184-188. 3 Russel-Jones,G.J., Gotschlich,E.C. and Blake, M.S. (1984) A surface receptor specific for human IgA on group B streptococci possessing the Ibc protein antigen. J. Exp. Med. 160, 1467-1475. 4 Schal~n, C., Truedsson, L. Christensen, P. and Christensen, K. (1985)Blockingof antibodycomplement-dependent effector functions by streptococcal IgG Fc-receptor and staphylococcalprotein A. Acta Path. Microbiol. Immunol. Scand. Sect. B, 93, 395-400.

54 5 Burova, L.A., Schal6n, C., Koroleva, I.V. and Svensson, M.-L. (1989) Role of group A streptococcal IgG Fc-receptor in induction of anti-lgG by immunization in the rabbit. FEMS Microbiol. Immunol. 47, 443-448. 6 Christensen, P. and Schal6n, C. (1990) Bacterial Fc receptors as putative virulence factors. In: Bacterial Immunoglobulin-Binding Proteins. Vol. I. (Boyle, M.D.P. Ed.), pp. 347-364. Academic Press, San Diego, CA. 7 Christensen, P., Burova, L.A., Grubb, A., Grubb R., Samuelsson, G., Schal6n, C. and Svensson, M.-L. (1979) Interaction of the Fc part of IgG with Lancefield extracts of hemolytic streptococci. Strain specificity and activity. Acta Path. Microbiol. Scand. Sect. C, 87, 73-77. 8 Raeder, R., Faulmann, E.L. and Boyle, M.D.P. (1991) Evidence for functional heterogeneity in IgG Fc-binding proteins associated with group A streptococci. J. lmmunol. 146, 1247-1253. 9 Otten, R.A., Raeder, R., Heath, D.G., Lottenberg, R., Cleary, P.P. and Boyle, M.D.P. (1992) Identification of two type IIa lgG-binding proteins expressed by a single group A streptococcus. J. Immunol. 148, 3174-3182. 10 Gomi, H., Hozumi, T., Hattori, S., Tagawa, C., Kishimoto, F. and Bj6rck, L. (1990) The gene sequence and some properties of protein H, a novel IgG-binding protein. J. Immunol. 144, 4046-4052. 11 Heath, D.G. and Cleary, P.P. (1989) Fc-receptor and Mprotein genes of group A streptococci are products of gene duplication. Proc. Natl. Acad. Sci. USA 86, 4741-4745. 12 Schal6n, C. (1980) The group A streptococcal receptor for human IgA binds IgA via the Fc-fragment. Acta Path. Microbiol. Scand. Sect. C. 88, 271-274. 13 Schal6n C., Christensen, P., Grubb, A., Samuelsson, G. and Svensson, M.-L. (1980) Demonstration of separate receptors for human IgA and IgG in group A streptococci type 4. Separation of the solubilized receptors from groupand typespecific antigens, lipoteichoic acid and peptidoglycan. Acta Path. Microbiol. Scand. Sect C, 88, 77-82. 14 Stenberg, L., O'Toole, P. and Lindahl, G. (1992) Many group A streptococcal strains express two different immunoglobulin-binding proteins, encoded by closely linked genes: characterization of the proteins expressed by four strains of different M-type. Mol. Microbiol. 6, 1185-1194. 15 Lindahl, G. (1989) Cell surface proteins of a group A streptococcus type M4. The IgA receptor and a receptor related to M proteins are coded for by closely linked genes. Mol. Gen. Genet. 216, 372-379. 16 Frithz E., Hed~n, L.-O. and Lindahl, G. (1989) Extensive sequence homology between IgA receptor and M proteins in Streptococcus pyogenes. Mol. Microbiol. 3, 1111-1119. 17 Lindahl, G., Akerstr6m, B., Vaerman, J.P. and Stenberg, L. (1990) Characterization of an IgA receptor from group B streptococci: specificity for serum IgA. Eur. J, Immunol. 20, 2241-2247. 18 Schal~n, C. (1992) The receptors for IgA in group A and

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