Immunoglobulin isotype influences affinity and specificity

Arturo Casadevall1 and Alena Janda Division of Infectious Diseases, Departments of Microbiology and Immunology and Medicine, Albert Einstein College of Medicine, Bronx, NY 10461

or almost half a century the Ig (CL) domain. These investigators also V region affinity and/or specificity (5, 7, molecule has been considered deleted the entire first domain of the 8, 10, 13), it is worthwhile to consider the F a bifunctional molecule consisting heavy chain (CH1) from an IgG2a Fab and profound implications of this phenomenon of two independent regions, a found significantly altered Ag binding. for humoral immunity. The ability of the variable (V) region responsible for speci- They suggested this was due to a confor- C region to influence specificity could ficity and affinity, and a constant (C) re- mational change in the CL domain upon help explain isotype restriction for certain gion responsible for such effector losing its heavy chain binding partner, Ab responses, such as the preference for functions as complement activation and which translated into a conformational IgM/IgG3 and IgG2a in murine responses interaction with Fc receptors. The origin change in the heavy chain (VH) and to polysaccharide Ags and viruses, re- of this view dates back to the Nobel Prize- light chain (VL) domains (6). spectively (14, 15). For example, a higher winning work of Rodney R. Porter, who Two years later, a comparison of bind- affinity or novel specificity found in an used proteolytic digestion in the late 1950s ing characteristics of a family of isotype isotype switched B cell could lead to to cleave the Ab molecule into fragments switched Abs to a microbial polysaccharide preferential binding and clonal expansion. that eventually were known as the antigen suggested that changing the isotype al- In addition, the Ab idiotype (Id) for V (Ag) binding fragment, or Fab, and the tered Ab–Ag binding kinetics and region identical mAbs has been shown to Fc domain, so named because it could be be affected by the choice of C region, easily crystallized (1). Two decades later, suggesting an explanation for isotype re- the elucidation of the mechanism for More focus on the role striction in anti-Id responses (16). The fact the generation of Ab diversity supported of Ig isotype may be that isotype switching can alter the affinity this concept by showing that distinct and/or specificity of an Ab implies that genes encoding V and C regions were re- critical in increasing the primary and secondary responses could arranged to express the Ig molecule (2). originate from different B-cell populations Additional evidence for the independence efficacy and application and that isotype switching could lead to between V and C region roles came from loss of reactivity for the original epitope the demonstration that isotype switching of immunoglobulins in while gaining novel specificity. In this re- produced new effector functions, while gard, it is noteworthy that isotype switch- preserving antibody specificity. Further- developing therapeutics ing of mAbs to fungal polysaccharide more, studies done by Oi et al. (3), using conferred reactivity with self-Ags (12), fluorescent labels, failed to provide any and diagnostics. potentially implicating this phenomenon evidence of interaction between the two in the generation of autoimmunity, regions. Consistent with this view, X-ray whereby autoreactive Abs also express crystallographic studies of Fab fragments thermodynamic parameters despite the isotype restriction (17). C region-mediated showed that V region sequences were presence of identical V region protein changes on V region structure could ex- separated from the first domain of the C sequences (7). Several years later, IgG1 plain the phenomenon of isotype restric- region (CH1) by long polypeptide chains and IgA mAbs, as well as their Fabs, all tion of Id responses and the immuno- that lacked ordered structure and seemed with identical V regions binding the same genicity and tolerance of certain Ids (18). to insulate the V regions from the C re- epitope, were found to manifest differ- The realization that the C region can in- fi gions, while tethering the two regions into ences in af nity (8). These observations fluence V region affinity and/or specificity one molecule (4). However, this neat view failed to change the existing dogma, pos- has important implications for the engi- fi of one molecule with two independent sibly because the eld was reluctant to neering of Ab molecules and the choice of functional regions is at odds with several abandon a cherished paradigm without isotype in therapeutic Abs. It may also observations in the literature, and recent additional data. Alternatively, it is possible need to be considered in creating more studies, including the study by Tudor et al. that a contribution from avidity differ- effective vaccines. in PNAS (5), suggest that the basic model ences was not unequivocally ruled out. The mechanism by which C region of Ig structure-function needs to be re- However, in the past decade, a series of affects V region affinity and/or specificity considered and revised. studies done with a family of V region is currently unknown. Given that C region identical isotype switched Abs to Crypto- can affect the Id, a plausible explanation C Region Can Affect V Region coccus neoformans polysaccharide using is that differing C regions allosterically Evidence that the C region can affect V monovalent peptide mimetics provided impose constraints on V region structure, region structure and translate into differ- strong evidence for C-mediated effects on paratope, and flexibility to differing ences in affinity and/or specificity has V region function as manifested by iso- degrees, upon binding Ag. Indirect evi- been accumulating for some time. In 1991, type-related differences in specificity and dence for this mechanism comes from Kato et al. (6) labeled specific residues affinity (9–12). In addition, two other circular dichroism studies showing the with 13C in three murine Fab isotypes— groups, including the report by Tudor IgG1, IgG2a, and IgG2b—and used NMR et al. (5), described additional examples in to study their positions before and after which Abs expressing identical V region Author contributions: A.C. and A.J. wrote the paper. Ag binding. The results revealed signifi- sequences manifested altered specificity The authors declare no conflict of interest. cant differences between these Fabs upon and/or affinity (13). See companion article on page 12680. fi Ag binding in the positions of two con- Given that ve independent groups 1To whom correspondence should be addressed. E-mail: served residues in the light chain constant have now reported that C region can affect [email protected].

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existence of a structural linkage between transcytosis at much lower concentrations entry pathway for HIV-1 infection, and the V and C domains that can affect V than IgG1. However, the IgG1 was more this study has important implications for region conformation upon binding Ag efficient in blocking HIV-1 transfer from vaccine design, suggesting the need to (19). It is noteworthy that not all isotype LCs to CD4+ T cells. Perhaps unex- elicit both mucosal IgA responses to changes result in V region changes (20), pectedly for Abs with similar specificity, HIV-1 infection as well as IgG responses, and it is conceivable that the susceptibil- the mAbs in combination synergistically because the synergistic effects of this ity of the V region to structural modifi- blocked HIV-1 transfer to autologous T combination are very powerful. cation by the attached C region is cells and reduced CD4+ T-cell infection. Finally, we note that the finding that afunctionoftheVregionusedand/or Finally, epitope mapping studies showed the C region can affect V region affinity somatic mutations that either facilitate that the 2F5 IgG1 recognized an addi- and/or specificity also has esthetic impli- or inhibit the transfer of structural in- tional epitope to that of the IgA2 on gp41. cations because the previously divided formation from C to V. Clearly, an entirely Because both the 2F5 IgG1 and IgA2 Ig molecule has now been made struc- new facet of Ig structure is now apparent, mAbs have three identical V (VL, CL, and turally whole again, and the mechanism and there is suddenly a tremendous VH) region domains, the difference in of class switch recombination is recon- amount of work to be done to better epitope mapping and Ag binding must ciled with somatic mutation, because understand this critically important host originate from the CH domains. These both impact the generation of Ab di- defense molecule. studies suggest separate roles as well as versity and rely on the same molecular combinatorial roles for the IgG and IgA machinery. This has very important C Region Can Affect Viral Neutralization response to HIV-1 infection. In this case, implications for the creation of more ef- In PNAS, Tudor et al. (5) explore the in- differences in C regions complement each fective vaccines and immunotherapies for fluence of the C region of two human other regarding the response to HIV-1 cancers and inflammatory and autoim- mAbs to HIV gp41: an IgG1 and a mono- infection, transcytosis, and neutralization. mune diseases. More focus on the role meric IgA2, on Ag binding, HIV-1 trans- This comparison of antiviral activity of two of Ig isotype may be critical in increasing cytosis, CD4+ T-cell infection and epitope mAb isotypes shows that activity is regu- the efficacy and application of immuno- mapping. The IgA2 bound native Ag lated and influenced by the C region. globulins in developing therapeutics tighter than the IgG1 and blocked HIV-1 Mucosal transmission is an important and diagnostics.

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