Attenuation of lymphoid leukosis enhancement by serotype 2 Marek's disease virus

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Abstract
Serotype 2 Marek's disease virus (MDV), already known to augment the protective efficacy of turkey herpes virus (HVT) vaccine against MD, may also enhance the frequency of lymphoid leukosis (LL) in retrovirus-infected chickens of certain strains. LL enhancement refers to an increase in frequency and decrease in latent period for development of LL, and is measured by comparison of tumour responses in retrovirus-infected, MD-vaccinated chickens to those of retrovirus-infected control chickens. LL enhancement has been documented with both of the serotype 2 MDV strains tested thus far. These experiments were conducted to investigate whether LL enhancement ability varied within a larger collection of serotype 2 strains or was influenced by serial passage in cell culture. Each of seven low-passage serotype 2 MDV strains enhanced LL responses. The HN-1 strain was competent for LL enhancement at passage 19, but LL enhancement ability was absent at passages 26, 27 and 40. Enhancement of LL by strains 471B/1, 281MI/1, 287C/1 and 298B/1 was reduced by 40 to 64 serial passages in chicken embryo fibroblast cultures. Strains SB-1 and 301B/1 continued to demonstrate enhancement of LL through 66 and 40 passages, respectively. Thus, LL enhancement appeared to be a general property of serotype 2 MDV, but was susceptible in five of seven strains to reduction (attenuation) by cell culture passage. Enhancement of LL by serotype 2 MDV was attenuated more rapidly by cell culture passage than was the ability of such viruses to protect against virulent MDV challenge either alone or in combination with HVT. One such LL enhancement-attenuated strain, 471B/1 (passages 33 and 40), when combined with HVT, induced protection against MD challenge (76 and 78%) that was comparable with that induced by SB-1 + HVT (82%) or 301B/1 + HVT (89%). This or similar serotype 2 viruses could provide suitable protection against MD with a reduced risk of LL enhancement.