CD4 is a receptor for Interleukin-16 (IL16), explaining how IL16 acts as a chemoattractant for a variety of CD4+ immune cells (Cruikshank et al. 2000, Cruikshank & Little 2008).
Signaling mediated by CD4 requires the amino acid sequence W345 to S350, located in the proximal end of the D4 domain. CD4 does not appear to require a co-receptor for IL16. Data from CD4 knockout mice suggests that there may be an additional IL16 receptor (Mathy et al. 2000).