High BTLA+CD4+T-lymphocyte percentage associated with poor critically ill ICU patient outcomes. Systemic inflammatory response syndrome (SIRS) patients that did not develop a subsequent nosocomial infection had a lower BTLA+CD4+ T-cell percentage compared with SIRS patients that developed a nosocomial infection and patients who were septic at the time of sampling (A) (histograms represent CD3+CD4+ gated lymphocytes). Specifically, a significantly diminished BTLA+CD4+ lymphocyte percentage was observed in the total SIRS population (n = 28) compared with the septic population (n = 11) (B) (dashed line, mean healthy volunteer BTLA+CD4+ lymphocyte percentage (n = 6); these patients were not sex or aged matched). Importantly, a significantly diminished BTLA+CD4+ lymphocyte level was also observed in the SIRS patients who did not develop a subsequent infection compared with those that did prior to hospital discharge (C). Additionally, septic patients that went on to develop a secondary nosocomial infection also maintained a high BTLA+CD4+ lymphocyte percentage (D). Regarding how the BTLA+CD4+ lymphocyte percentage associated with patient outcomes, the patients with >80% BTLA+CD4+ lymphocytes had a significantly longer hospital length of stay (LOS) (E). The BTLA+CD4+ lymphocyte percentage correlated moderately with the circulating CD4+ lymphocyte number in the evaluated total critically ill patient pool, suggesting that BTLA expression does not diminish the ability for CD4+ lymphocytes to enter circulation (F). Data are mean ± standard error of the mean. *P <0.05 using an unpaired, two-tailed Student’s t test. BTLA, B and T lymphocyte attenuator; R, Spearman coefficient following a nonparametric correlation test.