The major structural viral protein, VP1, of the human polyomavirus JC virus (JCV), the causative agent of progressive multifocal leukoencephalopathy (PML), was expressed by using recombinant baculoviruses. Recombinant VP1 formed virus-like particles (VLP) with the typical morphology of empty JCV capsids. Purified VP1 VLP bind to SVG, B, and T cells, as well as to monkey kidney cells. After binding, VP1 VLP were also internalized with high efficiency and transported to the nucleus. Immunization studies revealed these particles as highly immunogenic when administered with adjuvant, while immunization without adjuvant induced no immune response. VP1 VLP hyperimmune serum inhibits binding to SVG cells and neutralizes natural JCV. Furthermore, the potential of VP1 VLP as an efficient transporter system for gene therapy was demonstrated. Exogenous DNA could be efficiently packaged into VP1 VLP, and the packaged DNA was transferred into COS-7 cells as shown by the expression of a marker gene. Thus, VP1 VLP are useful for PML vaccine development and represent a potential new transporter system for human gene therapy.

Progressive multifocal leukoencephalopathy (PML) is a fatal, demyelinating disease caused by JC virus (JCV) in patients with severe immunosuppression. We studied the JCV-specific cellular and humoral immune response in 7 healthy donors (HD), 6 human immunodeficiency virus-1 (HIV-1)-infected patients without PML (HIV), 4 HIV-1-negative patients with PML (PML), and 8 HIV-1-positive patients with PML (HIV/PML). As antigens, recombinant virus-like particles of the major structural protein VP1 (VP1-VLP) of JCV, tetanus toxoid (TT), or the mitogen phytohemagglutinin (PHA) were used. Proliferation of peripheral blood mononuclear cells (PBMC) after stimulation with the VP1-VLP was significantly suppressed in PML and HIV/PML patients compared to HD. After antigen stimulation the production of interferon-gamma (IFN-gamma) was reduced in PML, in HIV/PML, and in HIV patients. The production of interleukin-10 (IL-10), however, was elevated in HIV/PML patients. Neither proliferation nor cytokine production correlated with the presence of JCV DNA in PBMC. The immunoglobulin G serum antibody titer to the VP1-VLP was slightly elevated in HIV, elevated in PML, and highly elevated in HIV/PML patients compared to HD. The development of PML appears to coincide with a general impairment of the Th1-type T-helper cell function of cell-mediated immunity.

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