Induction of central T cell tolerance: Recombinant antibodies deliver peptides for deletion of antigen-specific CD4+8+ thymocytes

DNA vaccines: MHC II-targeted vaccine protein produced by transfected muscle fibres induces a local inflammatory cell infiltrate in mice

Vaccination with naked DNA holds great promise but immunogenicity needs to be improved. DNA constructs encoding bivalent proteins that bind antigen-presenting cells (APC) for delivery of antigen have been shown to enhance T and B cell responses and protection in tumour challenge experiments. However, the mechanism for the increased potency remains to be determined. Here we have constructed DNA vaccines that express the fluorescent protein mCherry, a strategy which allowed tracking of vaccine proteins. Transfected muscle fibres in mice were visualized, and their relationship to infiltrating mononuclear cells could be determined. Interestingly, muscle fibers that produced MHC class II-specific dimeric vaccine proteins with mCherry were for weeks surrounded by a localized intense cellular infiltrate composed of CD45⁺, MHC class II⁺ and CD11b⁺ cells. Increasing numbers of eosinophils were observed among the infiltrating cells from day 7 after immunization. The local infiltrate surrounding mCherry⁺ muscle fibers was dependent on the MHC II-specificity of the vaccine proteins since the control, a non-targeted vaccine protein, failed to induce similar infiltrates. Chemokines measured on day 3 in immunized muscle indicate both a DNA effect and an electroporation effect. No influence of targeting was observed. These results contribute to our understanding for why targeted DNA vaccines have an improved immunogenicity.

Antibody-targeted vaccines

The specificity and high affinity binding of antibodies provides these molecules with ideal properties for delivering a payload to target cells. This concept has been commercialized for cancer therapies using toxin- or radionucleotide-conjugated antibodies that are designed to selectively deliver cytotoxic molecules to cancer cells. Exploiting the same effective characteristics of antibodies, antibody-targeted vaccines (ATV) are designed to deliver disease-specific antigens to professional antigen-presenting cells (APCs), thus enabling the host's immune system to recognize and eliminate malignant or infected cells through adaptive immunity. The concept of ATVs has been in development for many years, and recently has entered clinical trials. Early studies with ATVs focused on the ability to induce humoral immunity in the absence of adjuvants. More recently, ATVs targeted to C-type lectin receptors have been exploited for induction of potent helper and cytolytic T-cell responses. To maximize their stimulatory capacity, the ATVs are being evaluated with a variety of adjuvants or other immunostimulatory agents. In the absence of co-administered immunostimulatory signals, APC-targeting can induce antigen-specific tolerance and, thus, may also be exploited in developing specific treatments for autoimmune and allergic diseases, or for preventing transplant rejection. The successful clinical application of this new class of antibody-based products will clearly depend on using appropriate combinations with other strategies that influence the immune system.

Delivery of antigen to CD40 induces protective immune responses against tumors

Ligation of CD40 induces maturation of dendritic cells (DC) and could be a useful target for vaccines. In this study, we have constructed two types of Ab-based vaccine constructs that target mouse CD40. One type is a recombinant Ab with V regions specific for CD40 and has defined T cell epitopes inserted into its C region. The other type is a homodimer, each chain of which is composed of a targeting unit (single-chain fragment variable targeting CD40), a dimerization motif, and an antigenic unit. Such proteins bound CD40, stimulated maturation of DC, and enhanced primary and memory T cell responses. When delivered i.m. as naked DNA followed by electroporation, the vaccines induced T cell responses against MHC class II-restricted epitopes, Ab responses, and protection in two tumor models (myeloma and lymphoma). Two factors apparently contributed to these results: 1) agonistic ligation of CD40 and induction of DC maturation, and 2) delivery of Ag to APC and presentation on MHC class II molecules. These results highlight the importance of agonistic targeting of Ag to CD40 for induction of long-lasting and protective immune responses.