Sir, We read with interest the recent manuscript by Méndez-Lagares et al. 1 describing significant alterations in T cell homeostasis in patients on highly active antiretroviral therapy (HAART) showing low-level CD4 T cell repopulation, despite long-lasting and persistent viral replication control. In particular, notable reductions of circulating naive CD4 T cells with increased expression of markers for activation, senescence and proliferation were found in comparison with patients with satisfactory CD4 T cell repopulation. These results agree with their proposed model of immune impairment in patients with low-level CD4 T cell repopulation, 2 suggesting intrinsic thymus failure and specific features of premature immune senescence. Our laboratory is actively working on a custom eight-colour flow-cytometry assay (BD LyoTube 8-color CD4 and CD8 bundle, BD Biosciences, San Jose, CA, USA), which is able to analyse differentiation, activation and senescence in CD4 and CD8 T cells (CD4 Lyotube: CD95 FITC/CCR7 PE/CD3 PerCP-Cy 5.5/CD25 PE-Cy7/CD127 Alexa Fluor 647/CD45 APC-H7/CD4 AmCyan/CD45RA V450; clones DX2/150503/SK7/2A3/HIL-7R-M-21/2D1/SK3/HI100, respectively; and CD8 Lyotube: CD38

FITC/CCR7 PE/CD3 PerCP-Cy 5.5/CD69 PE-Cy7/CD127 Alexa Fluor 647/CD45 APC-H7/CD8 AmCyan/CD45RA V450; clones HB7/150503/SK7/L78/HIL-7R-M-21/2D1/SK1/HI100, respectively). Stained and lysed whole blood was analysed on an FACS-Canto II flow cytometer using FACS-Diva v. 6.1. 3 software (BD Biosciences). This test, performed on whole blood on a semiroutine scale, proved useful to analyse T cell homeostasis in HIV-infected persons in a protocol approved by the Institute’s Ethics Committee. In particular, this approach allowed us to verify that naive CD4 T cell frequency was lower in patients unable to reach CD4 reconstitution than in HIV patients reaching CD4 reconstitution [7.4%(IQR: 1.2–20.0) versus 33.1%(IQR: 27.9–43.9), P, 0.0001], 3 confirming the results of Méndez-Lagares et al. 1