NIH 3T3 Balb/c cells are transfected with 1.5 mg CIITA and 4.5 mL FuGene HD (7:2 ratio of FuGene to DNA for optimal cells growth) and selected for with L-histidionol for two weeks and analyzed for MHC II expressing using fluorescence-activated cell sorter (FACS) with PE-conjugated mouse IgG2a (Cedar Lane) against I-A d and clones were selected by limiting dilution. These cells were co-transfected with p32cDNA and pcHIV PAL (which contains all HIV genes except the LTR sequences) at a 2:1 ratio. The p32cDNA was donated by the Peterlin laboratory [59] . Both MHC II expressing VLPs and MHC II-negative VLPs were produced and concentrated 10-fold in a 100 K molecular weight cut-off filter tube (Millipore) for 15 minutes at 4000 rpm prior to titering via HIV-1 p24 CA Antigen Capture Assay Kit were performed following the manufacturer's instructions (NIH AIDS Research and Reference Reagent Program).