Breaking the affinity ceiling for antibodies and T cell receptors.

PNAS u September 26, 2000 u vol. 97 u no. 20 u 10679–10681 The antigen receptors made by lymphocytes are antibodies, which exist as soluble and cell-bound molecules, and Tcell receptors (TCRs), which are always found on cell surfaces. The function of these receptors in immunity depends on their specificity and affinity for antigen. Specificity—the potential to bind one unique chemical structure more strongly than a number of similar alternatives—is established by antibody and TCR gene rearrangements early in a lymphocyte’s ontogeny. Affinity—the equilibrium constant for antigen complexation (which we express as a dissociation constant, Kd)—can increase radically in the antibodyproducing B lymphocytes as a result of somatic hypermutation of antibody genes and selection of cells with improved binding phenotype (1, 2), an antigen-driven process known as affinity maturation. Although the extent of somatic hypermutation of rearranged TCR genes remains controversial (3), affinity maturation of T lymphocytes at the level of cell populations by selection of the available repertoire recently has been described (4). The affinities of antibodies and TCR obtained in vivo tend to fall within characteristic ranges constrained by biological requirements imposed during the ontogeny of B and T cells, about which more will be said later. Affinity maturation in vitro, as a rule, is not subject to the same biological constraints. In particular, affinity ceilings observed in vivo should not apply. The affinities of exhaustively in vitro matured antibodies or TCR should instead approach either a methodological ceiling intrinsic to the specific maturation protocol used or a molecular ceiling intrinsic to the architecture of antibodies and TCR. Two papers in PNAS describe in vitro affinity maturation of a TCR and an antibody using an ingenious system of yeast surface display (5, 6). In vitro TCR affinity maturation has not been reported previously. In vitro affinity maturation of antibodies has been described by several groups (7, 8), but never with so spectacular an endpoint, a Kd of 5 3 10214 M. In both the TCR and antibody cases the affinities ultimately obtained are orders of magnitude beyond the affinity ceilings observed in vivo. In this commentary we compare the in vivo affinity constraints with the in vitro constraints of the yeast system and comment on how breaking the affinity ceiling may affect basic immunology and immunotherapy.