T helper cell-mediated in vitro responses of recently and remotely infected subjects to a candidate recombinant vaccine for human parvovirus B19

T cell proliferation to human parvovirus B19 antigen was measured in 6 pati ents with recent B19 infection (1 with pneumonia and pleuritis), 1 patient with symptoms persisting >180 days after onset, 18 nonsymptomatic subjects with remote B19 immunity, and 12 B19-seronegative control subjects. Recombi nantly expressed virus-like particles (VP1/2 capsids), a candidate B19 vacc ine, were used as antigen. Virus-specific T helper cell proliferation was d etectable in all the recently infected patients and in most (17/18) of the remotely infected subjects but not in the seronegative control subjects. Th e B19-specific T cell responses, in general, were most vigorous among the r ecently infected patients. However, such strong B19-specific proliferation was not confined within the acute phase, as 28% (5/18) of the remotely infe cted healthy individuals had B19-specific reactivity persisting at acute-ph ase levels, apparently for years or decades. These data indicate that B cel ls recognizing the VP1/2 capsids receive class II-restricted help from CD4( +) T lymphocytes.