Table 2. Covariation between HLA allele charge and average CDR3 charge of HLA-associated TCRs for HLA positions frequently contacted by CDR3 amino acids in solved TCR:pMHC crystal structures.

MHC Class	Position*	Contact frequency†	Full CDR3		Non-germline CDR3‡		AAs§
			R-value	p-value	R-value	p-value
II-$\beta$	70	1.48	−0.47	3.3e-04	−0.52	6.1e-05	DEGQR
II-$\alpha$	64	1.09	−0.15	0.33	−0.07	0.64	ART
I	152	0.47	0.00	0.99	−0.04	0.72	AERTVW
I	151	0.46	0.08	0.50	0.06	0.59	HR
I	69	0.26	−0.13	0.28	−0.14	0.24	ART
I	76	0.21	−0.08	0.49	−0.14	0.25	AEV
I	70	0.12	0.02	0.86	0.08	0.50	HKNQS

*Only positions whose charge varies across alleles are included.

†Total number of CDR3 residues contacted (using a sidechain heavyatom distance threshold of 4.5 Å) divided by number of structures analyzed.

‡CDR3 charge is calculated over amino acids with at least one non-germline coding nucleotide.

§Amino acids present at this HLA position.