LLT1 and CD161 expression in human germinal centers promotes B cell activation and CXCR4 downregulation by Llibre, A, López-Macías, C, Marafioti, T, Mehta, H, Partridge, A, Kanzig, C, Rivellese, F, Galson, J, Walker, L, Milne, P, Phillips, R, Kelly, D, Freeman, G, El Shikh, M, Klenerman, P, Willberg, C