Protection and Synergism by Recombinant Fowl Pox Vaccines Expressing Multiple Genes from Marek's Disease Virus

Recombinant fowl poxviruses (rFPV) were constructed to express genes from serotype 1 Marek's disease virus (MDV) coding for glycoproteins B (gB1), C (gC), and D (gD) and tegument proteins UL47 and UL48, as well as genes from serotypes 2 and 3 MDV coding for glycoprotein B (gB2 and gB3). These rFPVs, alone and in various combinations, including combinations of fowl poxvirus (FPV)/gBs with turkey herpesvirus (HVT), were evaluated for ability to protect maternal antibody-positive (ab+) and -negative (ab-) chickens against challenge with highly virulent MDV isolates. The protective efficacy was also compared with that of prototype Marek's disease (MD) vaccines. No protection was induced in ab+ chickens by rFPV expressing gC, gD, UL47, or UL48. In contrast, the rFPV/gB1 construct protected about 23% of ab+ chickens against MDV challenge compared with 26% for cell-associated HVT. Levels of protection by rFPV/gBs of different MDV serotypes was highest for gB1, intermediate for gB2, and lowest for gB3. When rFPV/gB1 was combined with cell-associated HVT, protection was enhanced by an average of 138% compared with the best component monovalent vaccine, and the mean level of protection was 59% compared with 67% for the HVT+SB-1 bivalent vaccine. Relatively high protection (50%) and enhancement (200%) were also observed between rFPV/gB1 and cell-free HVT. These results suggest a specific synergistic interaction between rFPV/gB1 and HVT, possibly analogous to that previously described between serotypes 2 and 3 viruses. Levels of protection by rFPV/ gB1 alone or by bivalent rFPV/gB1+cell-associated HVT were similar to those of conventional cell-associated MD vaccines. However, the bivalent rFPV/gB1+cell-free HVT vaccine was clearly more protective than cell-free HVT alone and, thus, may be the most protective, entirely cell-free MD vaccine thus far described.