Thus, the data from ablation models cannot be interpreted without also taking into account the actual rather than predicted ablation patterns, the kinetics of deletion and regeneration, the effect on the remaining DC compartment and the role the depleted cell populations may play in immune homeostasis in the steady state. Models in which MHC alleles required for specific antigen presentation are expressed only by a defined DC
subset would overcome most, if not all, of the problems associated with DC immunization, ABT-888 price antibody targeting and ablation strategies. By retaining the entire complement of DC subsets with their normal transcriptional and biochemical programme, these models have the potential
to define DC biology in a physiological context. So far, this aim has been achieved only for radioresistant DC subsets, namely LCs. A number of published models have studied responses to LCs in MHC-disparate bone marrow (BM) chimeras in which LCs remain of host origin, whereas the majority of DDCs and cDCs are replaced [6, 8, 80-82]. The functional capacity of LCs can then be assessed using well-characterized TCR transgenic T cells whose specificity is restricted by an MHC allele encoded within the radioresistant host genome. MHC I-restricted models have made use of the fact that the Kbm1 mutant allele does not allow presentation of the ovalbumin (OVA) epitope to CD8+ OT-I TCR-transgenic T cells. In these models, OT-I stimulation capacity is restricted to LCs and radioresistant stromal cells of the H-2k host reconstituted with H-2Kbm1 BM . Peptide 17 The preservation of deletion of OT-I cells in response to skin-derived antigen has been interpreted as indicating that LCs can induce CD8+ T cell deletion in vivo, but the possibility that the effect was mediated via MHC I-expressing LN stromal Fossariinae cells cannot be excluded . In contrast, MHC II-dependent skin responses are effectively restricted only to LCs in MHC II-disparate chimeras,
as LN stromal cells do not express MHC II . Two groups have published results from such models. Allen et al. used wild-type hosts reconstituted with MHC II-knock-out (H2-Ab1–/–) BM and concluded that LCs were unable to support CD4+ T cell proliferation . However, reconstitution with MHC II-knock-out BM would generate an immune system in which tonic MHC II-dependent TCR signalling was deficient due to a lack of MHC II expression by the vast majority of DCs [83-86]. Such tonic TCR signalling is known to be critical for the maintenance of TCR sensitivity and responsiveness to activation, motility and memory generation within the CD4+ T cell compartment [87-90]. Thus the lack of CD4+ T cell response may have been due to the failure of most DCs to express MHC II, rather than an inability of LCs to support T cell proliferation under physiological conditions.