LAUSR

describing the use of monoclonal antibodies for determining the relative proportions of HLA-I conformational variants displayed by Luminex Single Antigen Beads (LSAB). They confirmed previous results showing that (1) LSAB display significant amounts of HLA-I that are not associated with beta-2-microglobulin (β2m) and/or peptide, as opposed with native and functional peptide-associated β2m-associated heavy-chains (pepA-β2aHC), (2) iBeads are largely devoid of β2m-free heavy-chains (β2fHC) but have lower pepA-β2aHC levels, (3) resting cells only express pepA-β2aHC, explaining why antidenatured HLA antibodies (anti-dHLA) rarely provide positive crossmatches and do not impair graft survival. Their findings are interesting but several issues could be discussed. First, the TFL-006 monoclonal antibody generated by the authors does not seem very efficient at staining the β2fHC that are present on LSAB. Indeed, TFL-006 provided lower mean fluorescence intensities (MFI) than HC-10 on acidtreated beads (that display only β2fHC, letter Figure 1), this later showing, as expected, comparable MFI with W6/32 on nontreated LSAB. In line with this, in patients' sera, we described anti-dHLA directed against all the beads which are not recognized by TFL-006. Therefore, assessing the fraction of β2fHC on HLA-I beads by comparing TFL-006 and W6/32 MFI and stating that HC-10 reactivity on iBeads is mainly due to peptide-free β2m-associated heavy-chains