In previous studies, we have demonstrated that very virulent plus Marek's disease viruses (vv+MDV) are highly immunosuppressive in commercial meat-type chickens. The specific objectives of this work were to evaluate if vv+MDV immunosuppression (MDV-IS) is induced by reduction of lymphocyte responsiveness and/or viability. Three experiments were conducted to (i) compare vv+MDV 686 with a partially attenuated 686-BAC; (ii) compare vv+MDV strains (648A and 686) with vMDV (GA) and vvMDV (Md5); and (iii) compare chickens vaccinated with Md5-BACΔMEQ and with CVI988 + HVT. In each experiment, spleens were collected at 28-30 days post infection and lymphocytes were isolated and investigated in three ways: their proliferative response to Concanavalin A (ConA) was analysed by MTT proliferation assay; cell death, and expression of CD45 and MHC-I was studied by flow cytometry; and MHC-IA and β-2 microglobulin (B2M) expression was evaluated by real time RT-PCR. Splenocytes of chickens inoculated with vv+MDV were severely impaired to proliferate when exposed to ConA. Furthermore, vv+MDV induced severe splenocyte death that did not occur after infection with v or vvMDV strains. Vaccination with CVI988 + HVT, and at less level with Md5-BACΔMEQ reduced these negative effects. This is in contrast to our previous results in which Md5-BACΔMEQ but not CVI988 + HVT protected against MDV-IS suggesting that although cell death and decrease lymphocyte function seem to be related to MDV virulence and certainly will be associated with immunosuppression, they might not fully explain the previously reported MDV-IS. RESEARCH HIGHLIGHTS vv+MDV induces extensive death in splenocytes in meat-type chickens 28-30 dpi. vv+MDV impairs lymphocyte function in meat-type chickens 28-30 dpi. Vaccination protects against splenocyte death and reduced lymphocyte function. Cell lysis and reduced lymphocyte function do not fully explain MDV-IS.