PDCD1 and HIV infectious disease: Interestingly, additional studies revealed that lymph node (LN) germinal center (GC) T follicular helper (Tfh) cells, co-expressing programmed cell death protein 1 (PD-1) and T cell immunoreceptor with Ig and ITIM domains (TIGIT), were functionally impaired and may provide an inadequate B-cell help during HIV infection [21–24].