CD4 and infection: FACS analysis on the blood 2 days after challenge showed that the CD3+/CD4+ cell population of the total CD3 population was between 0–0.23% in mice receiving GK1.5 compared to approximately 60% in non-depleted mice, indicating a very efficient depletion of the CD4+ T cell subset in treated animals (Fig. 5A) which was maintained throughout the infection period.