Dendritic cell responses to apoptotic cells : Is there life after death?

Dendritic cells (DC) are antigen-presenting cells that are crucial for the induction of immune responses to pathogens and for the maintenance of self-tolerance. DCs efficiently capture and process material from their local environment, including invading pathogens and dying cells of the host, and present the material to the adaptive immune response. Both in disease and during steady-state, cells die by apoptosis which lead to engulfment by surrounding DCs. Clearance of apoptotic cells by DCs has generally been considered to be an immunological silent event or to be involved in peripheral tolerance mechanisms. However recent discoveries suggest that apoptosis, under certain circumstances, can be immunogenic. In this work we find that the activation state of the cells undergoing apoptosis may determine whether exposed DCs will become activated and efficiently present antigen from these apoptotic cells to T cells. We demonstrated that activated, but not resting, apoptotic PBMCs induced increased expression of co-stimulatory molecules as well as release of pro-inflammatory cytokines in human DCs. Additionally, we showed that DCs exposed to allogeneic, activated, apoptotic PBMCs were able to induce proliferation and IFNgamma production in autologous T cells. We have also examined the ability of DCs to produce the Th1 promoting cytokine IL- 12p70 upon apoptotic cell uptake. We demonstrated that IL-12p70 production in DCs after uptake of apoptotic cells and subsequent CD40-ligation or IFNgamma/LPS stimulation was influenced by the activation state of the engulfed apoptotic cells. A CD40-ligand transfected cell-line induced IL-12p70 in DCs regardless of previous apoptotic cell uptake. However, IL-12p70 production by DCs in co-culture with allo-responsive autologous T cells required previous exposure to activated apoptotic cells. Resting, but not activated apoptotic cells reduced ongoing IL-12p70 production in DCs. This suggests that apoptotic cells may either license DCs for IL-12p70 production or dampen this ability depending on the activation state of the apoptotic cells. In addition, we have shown that syngeneic, activated apoptotic mouse lymphocytes were able to provide adjuvant activity in an HIV-1 DNA vaccine. Immunization of mice with seven different plasmids (3 env, 2 gag, 1 rev, 1 RT) combined with activated apoptotic cells lead to increased systemic and mucosal B cell responses as well as T cell responses in a magnitude comparable to immunization with plasmids and the cytokine adjuvant GM-CSF. Resting apoptotic mouse lymphocytes did not provide this adjuvant activity. Furthermore, we demonstrated that exposure to activated apoptotic CD4+ T cells promoted expression of co-stimulatory molecules, cytokine and chemokine release and reduced HIV-1 production in DCs. These effects were detected also with activated, newly infected apoptotic CD4+ T cells and remained in the presence of free HIV-1Bal. Blocking of TNFalpha decreased CD86 expression and partially restored HIV-1 production in DCs. In addition, up-regulation of HIV-1 interfering APOBEC3G was found in DCs exposed to activated apoptotic CD4+ T cells, which could contribute to the reduced HIV-1 production. Conclusively, our work demonstrates that the activation state of apoptotic cells influences DC activation and subsequent adaptive immune responses. We suggest that this could play a role in HIV-1 infection, where recurrent apoptosis is a distinctive feature, and also propose that the effects of activated apoptotic cells could be employed in design of vaccines