Alefacept is a fusion protein that contains the CD2-binding domain of LFA3 fused to human IgG1, and it is used to block CD2 function on human T cells. In addition, following in vivo administration,...

Alefacept is a fusion protein that contains the CD2-binding domain of LFA3 fused to human IgG1, and it is used to block CD2 function on human T cells. In addition, following
in vivo
administration, alefacept was also shown to deplete T cells of the effector or memory subsets that express high levels of CD2. Surprisingly, clinical trials data indicated that patients on alefacept still retained responses to vaccination. For example, one set of data showed that patients on alefacept made similar responses as the control group to a vaccine designed to protect against pneumococcal disease, which is composed entirely of bacterial polysaccharide antigens (pneumococcal polysaccharide vaccine; PPV). This normal response to PPV by patients on alefacept is not surprising because:

1. Alefacept is not very effective at blocking T cell responses.


2. CD2 is not required for all T cell responses to vaccination.


3. Alefacept does not deplete all effector or memory T cells, just a subset.


4. T cells are involved in responses to polysaccharide antigens.


5. The PPV was eliciting a recall response in previously exposed individuals.