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The Immune System: Strand Swapping for T-Cell Inhibition

SBKB [doi:10.1038/sbkb.2012.143]
Featured Article - June 2013
Short description: An immunoglobulin protein with a role in antigen-independent T-cell signaling dimerizes via a strand-swapped interface.

Ribbon diagram of the strand-swapped dimer of the IgV domain of mB7H3. Protomers are green and red, the β-strands yellow and the swapped G strand black. Reprinted with permission from Elsevier. 1

T-cell activation is dependent on the integration of both antigen-dependent signals through the T-cell receptor and antigen-independent interactions through ligands such as the B7 protein family.

Almo, Vigdorovich and colleagues (PSI NYSGRC) investigated the extracellular region of murine B7-H3 (mB7H3), a recently identified B7 protein implicated in various malignancies but of unclear precise function. Composed of a single IgV-IgC domain pair, mB7H3 was expressed in Drosophila cells as a glycosylated monomer. Notably, the authors obtained crystals only when using protein stocks that had been stored for several weeks and dimerized by a kinetically slow process. The 2.97-Å resolution structure (PDB 4I0K) revealed paired mB7H3 molecules with extensive interactions between the two IgV domains. Specifically, the segment connecting F and G strands (FG loop) of each IgV domain adopts an extended conformation, resulting in non-canonical strand swapping that mediates dimer formation. In contrast, the C-terminal IgC domain adopts the expected β-sandwich fold. Overall, the assembly spans ∼155 Å and the dimerization mode is distinct from that usually employed by the immunoglobulin superfamily.

The authors used the mB7H3 structure to construct a model for the monomeric form, which aligned favorably with PD-L1, a representative B7 structure. Next, they measured the effect of exogenously added mB7H3 to T-cell proliferation and observed a 50% decrease in the presence of freshly purified, monomeric wild-type mB7H3. Thus, mB7H3 can inhibit T-cell activation. Single alanine substitutions of FG loop residues had only minor effects on inhibitory activity, whereas replacement of the entire loop with the equivalent sequence from PD-L1 caused almost complete loss of activity, indicating that these residues interact with a T-cell receptor that remains to be identified.

The researchers further investigated the biophysical basis and functional effects of mB7H3 dimerization. Substitution of the four-residue sequence I(Q/R)DF in the mB7H3 FG loop with the equivalent, conserved YGGA sequence from PD-L1 significantly slowed dimerization. Furthermore, dimeric mB7H3 was similar to the monomeric form in its ability to inhibit T-cell proliferation. The authors note that, while such strand-swapped dimers are not unprecedented, their functional relevance has been unclear, as is the exact function of B7-H3. However, the functional importance of the FG loop residues represents an avenue for drug development.

Michael A. Durney

References

  1. V. Vigdorovich et al. Structure and T cell inhibition properties of B7 family member, B7-H3.
    Structure. 21, 1-11 (2013). doi:10.1016/j.str.2013.03.003

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