TLR4- and TRIF-dependent stimulation of B lymphocytes by peptide liposomes enables T cell–independent isotype switch in mice

Abstract

Immunoglobulin class switching from IgM to IgG in response to peptides is generally T cell-dependent and vaccination in T cell-deficient individuals is inefficient. We show that a vaccine consisting of a dense array of peptides on liposomes induced peptidespecific IgG responses totally independent of T-cell help. Independency was confirmed in mice lacking T cells and in mice deficient for MHC class II, CD40L, and CD28. The IgG titers were high, long-lived, and comparable with titers obtained in wild-type animals, and the antibody response was associated with germinal center formation, expression of activation-induced cytidine deaminase, and affinity maturation. The T cellindependent (TI) IgG response was strictly dependent on ligation of TLR4 receptors on B cells, and concomitant TLR4 and cognate B-cell receptor stimulation was required on a single-cell level. Surprisingly, the IgG class switch was mediated by TIR-domaincontaining adapter inducing interferon-␤ (TRIF), but not by MyD88. This study demonstrates that peptides can induce TI isotype switching when antigen and TLR ligand are IntroductionMost peptide and protein antigens require T-cell help for B-cell activation and antibody production, 1 while polysaccharides and lipopolysaccharide (LPS) can stimulate antibody responses without T help. 2 In both T cell-independent (TI) and -dependent (TD) antibody responses, cognate antigens bind their B-cell receptor (BCR). TD antigens are internalized, processed, and presented in the context of MHC class II molecules to antigen-specific T-helper cells. 3 The activated T cell then facilitates B-cell responses and immunoglobulin isotype switching via costimulatory molecules, adhesive antigens, and cytokines. TI antigens typically stimulate transient IgM antibody production with little or no IgG, IgA, or IgE production. TI type 1 (TI-1) antigens are polyclonal B-cell activators such as LPS whereas TI type 2 (TI-2) antigens consist of repetitive biochemical structures, such as polysaccharides or glycoproteins. 4 Liposomes are submicron vesicles of phospholipids and allow integration of compounds within and on the lipid bilayer. Antigens can be packed on the surface to resemble TI-2 antigens; in nature, repetitive arrangement of a single protein or peptide on cells or microorganisms does not typically occur. In the present study, a peptide was palmitoylated and mixed with phospholipids and monophosphoryl lipid A (MPLA) to allow formation of liposomes with densely arranged peptides on the outer surface. The palmitoylated human ␤-amyloid (A␤, aa1-15) peptide adopts an aggregated ␤-sheet conformation on liposomes. 5 While previous reports have suggested that nonreplicating protein vaccines do not enable TI isotype switching, 6,7 the present study in mice demonstrates that switch is feasible with a correct structural assembly of antigen and adjuvant. This result could pave the way for vaccination of patients with T-cell deficiencies or elderly, or when T-cell involvement is associated with immunopathology or autoimmunity, f...

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