Dendritic Cell Regulation of Peripheral Tolerance in Polyclonal T Cell Repertoires

Dendritic cells (DCs) play a pivotal role in determining whether the outcome of the immune system\u27s encounter with antigen will be immunity or tolerance. Using an antibody against the DEC-205 receptor, antigens have been delivered specifically to DCs in vivo. Under steady state conditions, such presentation of antigen leads to peripheral tolerance in transgenic T cells, either by deletion, anergy or the induction of regulatory T cells. We wanted to examine whether delivery of autoantigens to DCs using this approach, could be used to tolerize autoreactive polyclonal T cells, thereby preventing autoimmunity in mouse models. We succeeded in inducing tolerance to the myelin oligodendrocyte protein and preventing disease in the experimental autoimmune encephalomyelitis model. However no halt or delay in onset of autoimmune diabetes was observed when insulin was targeted to DCs in the non-obese diabetic (NOD) mouse model. Also, foreign antigen-specific T cell responses could not be abolished by targeting antigen to DCs in the NOD mouse. These results strongly suggested that establishing peripheral tolerance in disease-prone polyclonal repertoires such as in the NOD model, would be far more challenging than the previously studied tolerance in non-autoreactive transgenic models had been. We recognized that success in DC-targeting-based autoimmune therapy would first require a better understanding of tolerance in non-disease prone polyclonal T cell repertoires. Towards that end, we examined T cell tolerance in C57BL/6 mice. In the steady state, targeting ovalbumin (OVA) to DCs resulted in polyclonal CD4 and CD8 T cell tolerance. This tolerance was non-deletional and characterized by persistence of T cells that produced IFNγ, but no IL-2. CD4 dependent antibody production by B cells in vivo was abrogated. Also, both CD4 and CD8 proliferative responses in vitro were abolished. Subsequent to tolerization, depending on the strength of the costimulatory stimulus that the CD4 and CD8 T cells are exposed to, tolerance can be reversed both in vivo and in vitro. Thus our results demonstrate that while tolerance in non-autoreactive polyclonal repertoires in steady state DC environments can be achieved, reversal of the tolerized state can also occur. This suggests that in autoreactive T cell repertoires in chronically inflamed DC environments, the prevention or treatment of autoimmune disease is a challenge that will require comprehensive understanding of the balance between immunity and tolerance

Antigen targeting to dendritic cells elicits long-lived T cell help for antibody responses

Resistance to several prevalent infectious diseases requires both cellular and humoral immune responses. T cell immunity is initiated by mature dendritic cells (DCs) in lymphoid organs, whereas humoral responses to most antigens require further collaboration between primed, antigen-specific helper T cells and naive or memory B cells. To determine whether antigens delivered to DCs in lymphoid organs induce T cell help for antibody responses, we targeted a carrier protein, ovalbumin (OVA), to DCs in the presence of a maturation stimulus and assayed for antibodies to a hapten, (4-hydroxy-3-nitrophenyl) acetyl (NP), after boosting with OVA-NP. A single DC-targeted immunization elicited long-lived T cell helper responses to the carrier protein, leading to large numbers of antibody-secreting cells and high titers of high-affinity antihapten immunoglobulin Gs. Small doses of DC-targeted OVA induced higher titers and a broader spectrum of anti-NP antibody isotypes than large doses of OVA in alum adjuvant. Similar results were obtained when the circumsporozoite protein of Plasmodium yoelii was delivered to DCs. We conclude that antigen targeting to DCs combined with a maturation stimulus produces broad-based and long-lived T cell help for humoral immune responses

H2AX Is Required for Recombination Between Immunoglobulin Switch Regions but Not for Intra-Switch Region Recombination or Somatic Hypermutation

Changes in chromatin structure induced by posttranslational modifications of histones are important regulators of genomic function. Phosphorylation of histone H2AX promotes DNA repair and helps maintain genomic stability. Although B cells lacking H2AX show impaired class switch recombination (CSR), the precise role of H2AX in CSR and somatic hypermutation (SHM) has not been defined. We show that H2AX is not required for SHM, suggesting that the processing of DNA lesions leading to SHM is fundamentally different from CSR. Impaired CSR in H2AX−/− B cells is not due to alterations in switch region transcription, accessibility, or aberrant joining. In the absence of H2AX, short-range intra-switch region recombination proceeds normally while long-range inter-switch region recombination is impaired. Our results suggest a role for H2AX in regulating the higher order chromatin remodeling that facilitates switch region synapsis

Antigen targeting to dendritic cells elicits long-lived T cell help for antibody responses

Resistance to several prevalent infectious diseases requires both cellular and humoral immune responses. T cell immunity is initiated by mature dendritic cells (DCs) in lymphoid organs, whereas humoral responses to most antigens require further collaboration between primed, antigen-specific helper T cells and naive or memory B cells. To determine whether antigens delivered to DCs in lymphoid organs induce T cell help for antibody responses, we targeted a carrier protein, ovalbumin (OVA), to DCs in the presence of a maturation stimulus and assayed for antibodies to a hapten, (4-hydroxy-3-nitrophenyl) acetyl (NP), after boosting with OVA-NP. A single DC-targeted immunization elicited long-lived T cell helper responses to the carrier protein, leading to large numbers of antibody-secreting cells and high titers of high-affinity antihapten immunoglobulin Gs. Small doses of DC-targeted OVA induced higher titers and a broader spectrum of anti-NP antibody isotypes than large doses of OVA in alum adjuvant. Similar results were obtained when the circumsporozoite protein of Plasmodium yoelii was delivered to DCs. We conclude that antigen targeting to DCs combined with a maturation stimulus produces broad-based and long-lived T cell help for humoral immune responses