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HumanleukocyteIgG Fcreceptors Recently, increasing evidence has indicated a previously unrecognized molecular and functional heterogeneity of Fc receptors for IgG (FcR) on human leukocytes. Although many questions remain to be answered, a coherent and relatively simple picture of this heterogeneity appears to be emerging from the work of several laboratories. Three distinct classes of FcR - FcRI, FcRII and FcRio as summarized in Table 1 - have been defined by a host of criteria, including reactivity with specific monoclonal antibodies. All three classes of FcR appear on autoradiograms of SDS-acrylamide gels as broad bands of distinctive mobility. The polydispersity, in most cases, is attributable to glycosylation although the low affinity receptor (FcRio) band may be a poorly separable doublet ~. While precise molecular weight values assigned by various investigators differ somewhat, the mean molecular weight of FcRI is about 72kDa, that of FcRII about 40kDa, and FCRioranges from 50 to 70kDa. Only in the case of FcRI is affinity for monomeric ligand high enough to be measured easily. Ka is 108-109 M ~. This property has allowed the number of sites to be quantified by direct ligand binding methods. Monocytes bear 1-4 x 104 FcRI per cell; U937 cells exhibit somewhat fewer FcRI though culture in gamma interferon increases the number dramatically 27. The binding of monomeric ligand to FcRII or FCRio cannot be easily perceived by direct binding assays. Demonstration of these latter two FcR requires the presentation of ligand as a complex, either soluble IgG complexes or IgG-coated particles. The three receptors are expressed on distinctive and overlapping populations of cells. Preliminary data indicate that FcRI and FcRII are present on peritoneal and alveolar macrophages (C. Anderson et al., unpublished). Thus, FcRI seems to be expressed only on mononuclear phagocytes whereas FcRII has a much broader range of cellular expression, being present not only on mononuclear phagocytes but on all granulocytes so far evaluated, on platelets, and on B lymphocytes. FcRio has its own distinctive pattern of cellular expression: macrophages but not monocytes; neutrophils; eosinophils (sparce); and a group of cells referred to as L cells by Horowitz and Bakke~4 which includes natural killer (NK) cells, killer (K) cells, large granular lymphocytes (LGL) and T cells bearing IgG FcR (T~,). Several but not all combinations of these three FcR are expressed on the various cell types in their resting state. For example, macrophages bear all three FcR. B lymphocytes, platelets, and NK cells bear only one, either FcRII or FCRto; no cell has been described bearing only FcRI. Monocytes and granulocytes bear two of the three, either FcRI and FcRII in the case of monocytes or FcRII and FcRioon granulocytes. The specificity of the three FcR for human IgG subclasses seems fairly uniform, yet subtle differences are already apparent and may become more significant with further evaluation. Because of its high affinity for ligand,

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I Department of Medicine, Ohio State University, Columbus, OH 432 I0, USA; and 2Department of Medicine, University of Rochester Medical Center, Rochester,NY 14642, USA

Clark L. AndersonI and R. JohnLooney2 FcRI on U937 cells has been reasonably well studied. The rank order of affinity of this receptor for the subclasses is IgG1 = IgG3 > IgG4. IgG2 proteins, with one exception, do not bind. Several studies 9.28 indicate that the specificity of FcRI on monocytes is similar. However, it should not be assumed that the subclass specificity of each of these three receptors will be the same no matter where they are found. FcRII specificity has been evaluated only on platelets. According to the results of several studies, all subclasses of IgG can mediate platelet responses such as aggregation and release reactions 29-31. However, evaluation of the inhibition of oligomer binding under somewhat different conditions indicates that platelets bind IgG1 and IgG3 equally well and the other two subclasses less readily 1°. The human IgG subclass specificity of FcRio has not been well delineated; the many studies of neutrophil FcR specificity which had shown preferential binding of IgG1 and IgG3 will have to be re-evaluated in light of the new data showing two FcR on that cell type. The one study which evaluates FcRtoexclusively indicates that in detergent solution it binds to immobilized IgG1 but not IgG3 (Ref. 6). The specificity of these three human FcR for IgG subclasses of mouse origin, on the other hand, is quite distinctive - an observation of considerable experimental, and perhaps of some therapeutic, importance. FcRI binds IgG2a and IgG3 with high affinity, as high as human IgG1, whereas murine IgG2b and IgG1 bind 2-3 orders of magnitude less readily. However, these latter two subclasses, IgG2b and IgG1, seem to bind preferentially to FcRII, although to be detected binding must be enhanced by using low ionic strength conditions and/or ligand complexes. Judging by the murine sub-classes capable of mediating NK cell killing, Ig@2a binds more readily to FCRto than IgG2b; IgG1 has little or no affinity 21. Distinctive preferences of FcRI and FcRII for murine IgG subclasses underlie the capacity of the monocytes of all individuals (except for those of one family in Europe32) to mediate IgG2a anti-T3 Tlymphocyte proliferation whereas the monocytes of only a subgroup of individuals (the majority of Caucasians but a small minority of Asians33) mediate IgG1 anti-T3 responses. Recent evidence indicates that the high affinity.FcRI on monocytes is responsible for the IgG2a response whereas monocyte FcRII mediates the IgG1 response3. This functional dimorphism of FcRII, according to unpublished data (Anderson and Looney)is correlated with structural dimorphism manifested by isoelectric focusing patterns of the 40kDa FcR molecules. FcRI and FcRII are each recognized by a separate monoclonal antibody, FcRmab32 and mab IV3, respectively. FcRmab32 binds to a site on the 72kDa receptor distinct from the ligand binding site such that both antibody and ligand can be bound simultaneously. The antibody binds to FcRI on all cells so far examined. Mab IV3, on the other hand, blocks ligand binding and recognizes all FcRII except those on B lymphocytes (it ~1986, ElsevierSciencePublishersB.V.,Amsterdarn 0167- 4919/86/$02.00

Immunology Today, vol. 7, No. 9, 1986

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Table 1. HumanIgG Fcreceptors

Characteristic

FcRI

Molecule

72 kDa

Ref.

FcRII

Ref.

FcRioa

Ref.

1,2

40 kDa

2-4

50-70 kDa

5,6

Affinity for IgG monomer Ka= 108-109M-1

7-9

monomer binding undetectable

3,10

monomerbinding undetectable

9

Sites/Cell

1-4 x 104

7-9

ND

Cells

monocytes (HL60,U937)

1,11

monocytes,neutrophils, 2-4,12, neutrophils,eosinophils, 2,5,6, eosinophils,platelets, B cells 13 macrophages,NK, K, LGL, 14-17 (U937, HL60, T,/ K562, Daudi, Raji)

Specificity, human(h)

hlgG1 = hlgG3 > hlgG4; 7,9 hlgG2 no

Specificity, murine(m)

ND

10,18

hlgG1= hlgG3

mlgG2a= mlgG3 mlgG1, 7,19,20 mlgG1 > mlgG2b mlgG2b no

3,13

mlgG2a>mlgG2b;mlgG1 21

anti-T3 proliferation

mlgG2a(OKT3)

3

mlgGl(Leu-4)

3

N/A

monoclonal antibodies

32

11

IV3

3,4,12

3G8,4F7,VEP13,Leu-11a, 5,16,17, Leu-1lb, B73.1 22-26

a

hlgG1 = hlgG3> hlgG2 and hlgG4

6,9

no

Io = low affinity

binds to Daudi cells). Another monoclonal antibody (KuFc79) has been described which recognizes p40 on B lymphocytes, monocytes, and granulocytes but not on platelets 34. Whether KuFc79 also recognizes the 72kDa FcR as claimed seems uncertain because, in our hands, any ascites fluid-derived IgG2b linked to SepharoseProtein A will adsorb FcRI (Ref. 4), presumably due to contaminating IgG2a and IgG3. FcRio is recognized by an array of antibodies, listed in Table 1, that seem to bind at least two distinct epitopes on the FcR~oof neutrophils and NK cells 23. Three lines of evidence suggest that FcRI and FcRII are the human homologues of the murine macrophage IgG2a receptor (FcRI) and the IgG2b/1 receptor (FcRII), respectively (murine terminology suggested by Unkeless35). First, in both species FcRI shows preferential affinity for murine IgG2a while FcRII shows preference for murine subclasses IgG2b and IgG1 (Ref. 36-38). Further, the affinity of FcRI for monomeric ligand is considerably higher than the affinity for monomer of the other two receptors, setting it off as a separate class. Second, the receptors are expressed on the same sets of cells. FcRII in both species is present on B lymphocytes, neutrophils, eosinophils, and on mononuclear phagocytes, but is not found on non-B lymphocytes (with the exception of some cell lines) 35 ' 39 •4 0 . This pattern, therefore, is quite distinctive from that of FCR~oin man. FcRI in both species is present only on mononuclear phagocytes 41. Third, FcRI is relatively trypsin sensitive in situ whereas FcRII is relatively trypsin resistant in both species 36,38. Despite these similarities between murine and human FcR, significant gaps in our knowledge limit the extent to which homologies can be drawn. For example, no routine homologue of FCRio has yet been described. Further, while the murine FcR which binds murine IgG3coated RBC (Ref. 42) seems to have no human correlate, the murine FcRI does not bind murine IgG3 as does FcRI in man. It has been suggested that whereas the capacity to bind murine IgG2a and IgG3 in man resides in a single

(presumed primordial) receptor, FcRI, the mouse has evolved two distinct FcR for each of the two isotypes, FcRI for IgG2a and a separate FcR for IgG3 (Ref. 19). Limiting the homologies still further is the observation that the murine and human FcRI bind different parts of the murine IgG2a molecule 43. Possibly because the molecules are so heavily glycosylated, no strict correlations are yet apparent between the molecular weights of the receptors of the two species. These comparisons will have to await studies of the non-glycosylated molecules. Despite these exceptions, the similarities noted above have prompted our use of the nomenclature adopted in Table 1. The designation of FCRto, proposed by Fleit et al. 44, we have not altered. All three of these FcR appear to show structural or functional variability within the population. Recent evidence indicates that the murine FcRII is the same molecule as the polymorphic alloantigen Ly17, the genetic locus for which is inseparable from the locus (MIs) responsible for mixed lymphocyte stimulation in the mouse 39,4°. Moreover, not only has allotypic variability been found on the FcRio molecule of human neutrophils 4s, but, as mentioned above, individuals have been described whose FcRI or FcRII do not mediate anti-T3 T-cell proliferation. Further, both diseaseassociated and HLA-linked abnormalities of FcRmediated endocytosis have been noted in man 46'47. The details and significance of these polymorphisms may be partially illuminated in the very near future once the FcR genes become available for study. We thank Drs J.P. Leddy and S.I. Rosenfeld for critically evaluating the manuscript. References

1 Anderson, C.L. (1982)J. Exp. Med. 156, 1794-105 2 Cohen, L., Sharp, S. and Kulczycki, A. Jr (1983) J. ImmunoL 131,378-383 3 Looney, R.J.,Abraham, G.N. and Anderson, C.L. (1986)

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J. Immunol. 136, 1641-1647

4 Rosenfeld, S.I., Looney, R.J., Leddy, J.P. etal. (1985)J. Clin. Invest. 76, 2317-2322 S Fleit, H.B., Wright, S.D. and Unkeless, J.C. (1982) Proc. Natl Acad.Sci. USA 79, 3275-3279 6 Kulczycki, A., Jr. (1984) J. Immunol. 133,849~354 7 Anderson, C.L. and Abraham, G.N. (1980)J. Immunol. 125, 2735-2741 8 Fries, L.F., Hall, R.P., Lawley, T.J. etal. (1982) J. Immunol. 129, 1041-1049 9 Kurlander, R.J. and Batker, J. (1982)J. Clin. Invest. 69, 1-8 10 Karas, S.P., Rosse, W.F and Kurlander, R.J. (1982) Blood 60, 1277-1282 11 Anderson, C.L., Guyre, P.M., Whitin, J.C. etal. (1986) J. Biol. Chem. (in press) 12 Looney, R.J., Ryan, D.H., Takahashi, K. etal. (1986) J. Exp. Med. 163, 826-836 13 Jones, D.H., Looney, R.J. and Anderson, C.L. (1985) J. Immunol. 135, 3348-3353 14 Horwitz, D.A. and Bakke, A.C. (1984) Immunol. Today 5, 148-153 15 Titus, J.A., Sharrow, S.O. and Segal, D.M (1983) J. Immunol. 130, 1152-1158 16 Lanier, L.L., Kipps, T.J. and Phillips, J.H. (1985)J. Exp. Med. 162, 2089-2106 17 Perussia, B., Trinchieri, G., Jackson, A. etal. (1984)./. Immunol. 133, 180-189 18 Dickler, H.B. (1977)Adv. Immunol. 24, 167-214 19 Perussia, B., Dayton, E.T., Lazarus, R. etal. (1985) J. Exp. Med. 158, 1092-1113 20 Lubeck, M.D., Steplewski, Z., Baglia, F. etal. (1985) J. Immunol. 135, 1299-1304 21 Kipps, T.J., Parham, P., Punt, J. etal. (1985)J. Exp. Med. 161, 1-17 22 Abo, T. and Balch, C.M. (1981)J. Immunol. 127, 10241029 23 Perussia, B., Start, S. and Abraham, 5. (1983)J. Immunol. 130, 2133-2141 24 Lanier, L.L., Le, A.M., Phillips, J.H. etal. (1983) J. Immunol. 131, 1789-1796 2S Rumpolt, H., Kraft, D., Obexer, G. etal. (1982) J. Immunol. 129, 1458-1464

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26 Phillips, J.H. and Babcock, G.F. (1983) Immunol. Lett. 6, 143 27 Guyre, P.M., Morganetli, P.M. and Miller, R. (1973)J. Clin. Invest. 72, 393-397 28 Lawrence, D.A., Weigle, W.O. and Spiegelberg, H.L. (1975) J. Clin. Invest. 55, 368-376 29 Pfueller, S.L. and L~scher, E.F. (1972)J. Imrnunol. 109, 517-525 30 Henson, P.M. and Spiegelberg, W.L. (1973)J. Clin. Invest. 52, 1282-1288 31 Martin, S.E., Breckenridge, R.T., Rosenfeld, S.I. etal. (1978) J. ImmunoL 120, 9-14 32 Ceuppens, J.L., BIoemmen, FJ. and Van Wauwe, J.P. (1985) J. Immunol. 135, 3882-3886 33 Abo, T., Tilden, A.B., Balch, C.M. etal. (1984) J. Exp. Med. 160, 303-309 34 Vaughn, M., Taylor, M. and Mohanakumar, T. (1985) J. ImmunoL 135, 4059-4065 35 Unkeless, J.C. (1979) J. Exp. Med. 150, 580-596 36 Heusser, C.H., Anderson, C.L. and Grey, HM. (1977)J. Exp. Med. 145, 1316-1327 37 Unkeless, J.C. and Eisen, H.N. (1975)J. Exp. Med. 142, 1520-1533 38 Unkeless, J.C. (1977)J. Exp. Med. 145,931-947 39 Holmes, K.L., Palfree, R.G.E., Hammerling, U. etal. (1985) Proc. Natl Acad. Sci. USA 82, 7706-7710 40 Mark, W.H., Kimura, S. and H~mmerling, U. (1985) J. Immunol. 135, 2635-2641 41 Unkeless, J.C., Fleit, H. and Mellman, I.S. (1981)Adv. Immunol. 31,247-270 42 Diamond, B. and Yelton, D.E. (1981)J. Exp. Med. 153, 514 43 Raychandhuri, G., McCool, D. and Painter, R.H. (1985) Mol. ImmunoL 22, 1009-1019 44 Fteit, H.B., Wright, S.D., Durie, C.J. etaL (1984)J. Clin. Invest. 73, 516-525 45 Werner, G., Krvon dem Borne, A.E.G., Bos, M.J.E. etal. (1985) in Leukocyte Typing II Vol. 3 Human myeloid and hematopoieticcells. (Reinherz, E.L., Haynes, B.F., Nadler, L.M. et aL eds) Springer Verlag, New York 46 Lawley, T.J., Hall, R.P., Fanci, A.S.etal. (1981) New Engl. J. Med. 304, 185 47 Salmon, J.E., Kimberly, R.P., Gibotsky, A. etaL (1986) J. Immunol. 136, 3625-3630