Presented by Mis Karimi (PhD student of medicine immunology)

Toll-like receptor (TLR) family

B-cell expression of TLRs  TLR expression patterns and their functions in different B-cell subsets  Expression of TLR on B lymphocytes varies depending on the mammalian species

Toll-like receptors in human B cells  Naïve human B cells do not express significant levels of TLRs unless they are pre-stimulated through the BCR  human memory B cells constitutively express TRL2, TLR6, TLR7, TLR9 and TLR10.  As in humans, TLRs are expressed differentially in B cell subsets.

Roles of TLR in activation of B cells

TLR SIGNALING IN B-CELL DEVELOPMENT  Activation of TLR signaling in HSCs shifts their developmental potential towards myeloid cells  TLR signaling in CLPs causes cell differentiation into DCs  Transitional B cells can differentiate into mature B cells, plasma cells and IgM memory B cells upon TLR activation

Cellular targets of TLR stimulation in human B cells  Interestingly, in the human body the local environment seems to shape the TLR repertoire  Memory B cells are more reactive and more prone to proliferate and differentiate into plasma blasts upon TLR stimulation  CpG-containing ODN, it is becoming more and more evident that only a few B-cell subsets respond to TLR9 stimulation with CpG-ODN

 Furthermore, CpG-ODN supported productive CSR in naive human B cells in the presence of recombinant interleukin-10  In a recent report Rita Carsetti’s group characterized transitional human B cells as the main non-memory B-cell subset responsive to TLR9 activation  Recently, studies have described human IgM+ CD27+ B cells as the main target cells for immunostimulatory DNA ODN

Models for Toll-like receptor (TLR) -mediated B-cell activation.

TLR signaling in B-cell differentiation  Toll-like receptors were proposed to play a role in B- cell differentiation at early stages of B-cell development  B-cell precursor maturation in vitro is supported by the early exposure to TLR4 ligands but arrested upon stimulation of TLR2  contrast, there is strong evidence that TLR regulate the differentiation of transitional and mature B cells.  TLR-mediated activation of the transcription factor NF-kB is believed to be sufficient to promote the expression of Blimp-1, a transcription factor crucial for plasma cell and pre-plasma memory B-cell differentiation.

TLR signaling in B-cell differentiation  Additionally, a recent study suggested a role for TRAF3 in TLR mediated and antigen-mediated plasma cell differentiation and IgG secretion.  There are no publications available on the persistence of TLR expression and the role of TLR in plasma cells.

Modulation of antibody production in vivo  TLR engagement in B cells mediates TI and TD antibody responses  Dual BCR and TLR engagement in antigen-specific B cells induces T cell-independent CSR or primes B cells for T cell-dependent CSR and SHM

TI and TD antibody responses

Dual BCR and TLR engagement in CSR and SHM

Modulation of CSR in vitro  In murine B cells the TLR4 ligand LPS promotes the CSR of naive B cells to IgG2b and IgG3, and to IgG1 and IgE when combined with IL-4.  Additionally, in murine B cells, TLR9 stimulation can inhibit LPS + IL-4- mediated CSR to IgG1 and IgE and promote IgG2a, IgG2b and IgG3 production

Modulation of CSR in vitro  In the human immune system Andrea Cerutti’s group was the first to find that CSR to IgG is induced when naive B cells are cultured in the presence of IL-10 and stimulated with CpG-ODN.  CSR to IgG and IgA was observed when human B cells isolated from tonsils were challenged with TLR3 ligand Poly I : C and BAFF.

Modulation of CSR in vitro  Moreover, the same group reported that T-cell-independent CSR to IgA2 is most efficient in human B cells treated with the TLR5 ligand flagellin or TLR9 ligand CpG DNA in the presence of APRIL.

TLR SIGNALING IN AUTOANTIBODY PRODUCTION  Marshak-Rothstein’s group first demonstrated that a synergistic engagement of BCR and TLR9 signaling could effectively activate autoreactive B cells in vitro.  Genetic deficiency of TLR7, TLR9 or MyD88 often leads to reduced production of autoantibody, and increased expression of TLR7 causes susceptibility to autoimmune diseases.

TLR SIGNALING IN AUTOANTIBODY PRODUCTION  Shlomchick’s group generates a model of SLE, they found that the autoreactive B cells actually proliferate and undergo somatic hyper mutation in the so-called ‘extra follicular’ sites, not in GCs.  When used B cell MyD88-deficient mice crossed to a different SLE mouse model (lyn-/-) and found a strong dependence on B-cell TLR signaling for autoantibody level and associated pathological change.

TLR SIGNALING IN ANTIBODY RESPONSE TO IMMUNIZATION  In cases of antigens containing defined TLR ligands, TLR signaling does play important roles in many aspects of antibody response.  For proteins conjugated with TLR ligands, which are typical TD type antigens, the antigen-specific IgM response is also dependent on B- cell TLR signaling to some degree

TLR SIGNALING IN ANTIBODY RESPONSE TO IMMUNIZATION Immunization mice with several soluble protein antigens with variable levels of immunogenicity mixed or directly conjugated to a TLR ligand and found that the magnitude of the IgG responses was comparable between B cell MyD88-deficient mice and wild-type controls Working model of how VLPs induce B- cell TLR signaling

TLR-induced cytokine secretion in B cell  Murine IL-10 secretion was detected in MZ B cells in response to single TLR2, TLR4, TLR9 or combined TLR stimulation but was absent in splenic follicular B cells whereas IL-6 was detectable in both populations.  Among the murine B-cell subpopulations tested, only follicular B cells synthesized IFN-γ in response to stimulation with TLR9 ligands combined with TLR2, TLR4 or TLR7 agonists.

TLR-induced cytokine secretion in B cell  Several reports further indicate that murine B cells produce TNF, IL-6, IL-12, IL-23 and IL-27 in response to TLR engagement.  Human B cells are generally considered poor cytokine producers. Stimulation with TLR7 and TLR9 ligands results in the secretion of IL-10, IL-6 and IL-8. Further, IL-1β and IL-2 secretion have been detected in response to CpG-ODN.

The role of BCR and MYD88 signaling in B cells during T-dependent immune responses.

Conclusion:

Perspective Understanding the mechanisms of how innate signaling contributes to humoral responses will provide a platform for developing applications, including vaccines, for human diseases TLRs are a potential target for therapeutic intervention in autoimmune diseases.

Thank you for attention

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