toodistantfromthepMHCtoformdirect
contacts in the bound state to mitigate se-
lecting for affinity-matured TCRs.
On the basis of the structure of the TCR55-
HIV-B35 complex ( 11 ), three residues on the
TCR55achain and four residues on the TCR55
bchain were selected for the library positions
(Fig. 1C). Our library consisted of mainly
charged and polar residues including glutamine,
glutamate, asparagine, aspartate, arginine, ly-
sine, serine, and histidine to increase the chances
of forming adventitious polar interactions.
The three randomized residues on the TCR55
achain were combined as one library with a
diversity of 1728 muteins (Valibrary), and the
four randomized residues on TCR55bchain
were combined as a second library with diver-
sity of 20,736 muteins (Vblibrary). Full-length
TCR55 libraries were synthesized and cloned
into a lentiviral backbone vector. Lentivirus
libraries were constructed and used to infect
the SKW3 T cell line at low multiplicity of
infection (MOI), and TCR libraries were ex-
pressed on the surface of T cells. The Vali-
brary was paired with the wild-type (WT)
TCR55bchain, and the Vblibrary was paired
with the WT TCR55achain in the transduced
SKW3 cells. The libraries were stimulated with10 mM HIV peptides and sorted for pMHC
tetramer staining–low (no higher than the
pMHC tetramer staining of WT TCR55) to-
gether with costaining for activation antigen
CD69-high [top 5% population based on anti-
CD69 mean fluorescence intensity (MFI)]
populations to enrich for low-affinity, high-
potency TCR mutants (Fig. 1D).SingleÐamino acid substitutions in TCR55
trigger activation through catch bond formation
We carried out three rounds of fluorescence-
activated cell sorting (FACS) selections on the
TCR55aCDR library (diversity: 1728) and en-
riched a population with a tetramer-low, CD69-
high staining phenotype (Fig. 2A and fig. S3,
A and B). Approximately 100 single-cell clones
were recovered and individually tested for ac-
tivation by the HIV(Pol) peptide. The two clones
(clone 8 and clone 17) that showed the most-
potent responses to this pMHC ligand (fig. S3C)
encoded identical TCR mutations on the TCR55
achain—S28G and A98H. To directly examine
whether the identified mutations conferred in-
creased potency, SKW3 T cells were transduced
with the TCR55a–S28G A98H and WT TCR55
bchain and stimulated by B35-associated HIV
peptide (fig. S3D). To deconvolute which muta-
tion was responsible for the activation, we
tested the mutations individually (fig. S3, D
and E) and found that the single mutation of
alanine to histidine in the TCR55aCDR3 was
sufficient to endow the nonresponsive TCR55
with the ability to be activated upon exposure
to the B35-HIV pMHC (Fig. 2B and fig. S4).
The 3D affinity of TCR55a-A98H binding
to the B35-HIV pMHC was measured by sur-
face plasmon resonance (SPR) asKD(3D bind-
ing affinity) = 5.9mM, which is approximately
threefold lower than the WT TCR55 bind-
ing to B35-HIV (KD= 17mM) but is still in
the physiological affinity range for TCR-pMHC
interactions and is higher than that mea-
sured for the binding of TCR589 to B35-HIV
(KD=4mM), a receptor-ligand pair with agonist
qualities (Fig. 2C and fig. S3F) ( 11 ). Biomembrane
force probe (BFP) experiments were conducted
to determine whether TCR55a-A98H forms
catch bonds with B35-HIV. The nonresponsive
WT TCR55 showed progressively shorter bond
lifetime with increasing force, which is con-
sistent with slip bond formation. By contrast,
application of force increased bond lifetime
between TCR55a-A98H and B35-HIV, indicat-
ing catch bond formation (Fig. 2D). Analysis of
the previously published structure of TCR55
bound to B35-HIV ( 11 ) suggests that the resi-
duesQ65andT69ontheB35MHCheavychain
molecule might form bonds with H98 on
TCR55a(fig. S3G). Q65 or T69 was mutated to
alanine, and only the Q65A mutation substan-
tially abrogated the activation of TCR55a-
A98H, which suggests that the triggering
catch bond may involve an interaction betweenZhaoet al.,Science 376 , eabl5282 (2022) 8 April 2022 2 of 14
Fig. 1. The design of catch bond fishing libraries and selection strategy.(A) TCR55-transduced SKW3
T cells were stimulated by KG-1 cells pulsed with titrated HIV or Pep20 peptides for 14 hours. Anti-CD69
staining was performed on the SKW3 T cells and analyzed by flow cytometry. (B) TCR55-transduced SKW3
T cells were stimulated by KG-1 cells pulsed with titrated HIV or Pep20 peptides for 15 min. AntiÐphospho-
ERK staining was performed on the SKW3 T cells and analyzed by flow cytometry. (C) The design of
TCR55 libraries. Each library has three or four residues selected to be randomized. The side chains of the
residues selected for mutation on TCR55 are shown as sticks in the figure. (D) Workflow of catch bond
engineering of TCR. [(A) and (B)] Data are representative of three independent experiments. Data are shown
as means ± SDs of technical triplicates. APC, antigen-presenting cell; SAv, streptavidin; PE, phycoerythrin.
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