Live, not dead bacteria, augments human antibody response via TLR8 - T follicular helper cell axis

Immune system protects against pathogens. In natural settings, live pathogens cause diseases. So, it is very intuitive to think that immune system could have developed a specialized way to detect pathogen's "live signature". Few years back, Julie Magarian Blander's lab (former postdoctoral scientist in Ruslan Medzhitov' lab) published study showing that mRNA from live bacteria served as "live signature" they called vita-PAMP. 

Now, new paper in nature immunology from Leif Sander's lab (a former postdoctoral scientist in her lab and the first author of initial study) showed that in humans TLR8 may serve as a detector of vita-PAMP mRNA from live bacteria and augment antibody response. It is very nice study done primarily on ex vivo/in vitro human cells cultures (interestingly, Blander's lab also published this month new study about vita-PAMPs in mice in journal Immunity. But it is very messy and bloated study. Basically, in this case pupil outdid his [former] master).

Here the authors showed that live, replication-defective E. coli strain but not heat-inactivated dead one, could induce differentiation of human follicular helper T cells (T_FH cells).

These T_FH cells were functionally active inducing antibody-secreting plasmablast generation from B cells.

Live bacteria, not dead one, specifically induced IL-12p70 generation from human monocytes (unlike mouse, differentiation of human T_FH cells requires IL-12p70).

Indeed, antibody blockade confirmed a major role of IL-12p70 in generation of IL-21-producing human T_FH cells.

Stimulation of human monocytes with various TLR agonists showed that engagement of TLR8 (single-strand RNA sensor) with its agonist ligand (CL075 or R848) were responsible for vita-PAMP effect on IL-12p70 production.

Similar vita-PAMP effect of TLR8 signaling were seen for human IL-21-producing T_FH cell generation.

In summary, this study indicates that unlike LPS-derivative MPLA or CpG fortified vaccines, inclusion of TLR8 agonists, such as CL075, could augment antibody responses. While study is well done we need to keep in mind that it is produced by members of the 'same initial' group (Blander and Sander) who first reported vita-PAMP effect. We don't have analogous studies from other labs who can independently confirm these observations. 

posted by David Usharauli

  

Why anti-viral immune serum doesn't work in every infected patient?

Many viruses, such as Ebola or avian-origin Flu viruses, are extremely virulent and can cause death of infected individuals within days. However, some individuals are naturally resistant to such virulent infections and generate protective, neutralizing antibodies in response to them. Many organizations across world responsible for public health safety [and also military institutions] try to stockpile such anti-viral sera obtained from individuals who survive natural infection. It is believed that injection of anti-viral serum into infected individuals could help them to survive.

However, new study in journal Nature suggests that anti-viral serum effectiveness depends on viral tropism. It turns out that protection against viruses that infect so called immune privileged tissues, such as brain tissue, required presence of both anti-viral neutralizing antibodies and anti-viral CD4 T cells. Specifically, the authors, led by Akiko Iwasaki from Yale School of Medicine, showed that anti-viral CD4 T cell's role is to open the "gate" to privileged tissues for anti-viral antibodies that usually can not cross such barrier on their own.

For this study the authors used herpes simplex virus type 2 (HSV-2) virus challenge model that infects mouse immune privileged tissues such as the innervating neurons in the dorsal root ganglia (DRG). Initially the authors reported and re-confirmed that if mice were vaccinated with attenuated strain of HSV-2 before WT virus challenge, then these vaccinated mice were protected against WT virus challenge and this protection depended on antibody.

Strangely, however, immune serum could not protect naive, non-vaccinated mice against WT virus challenge. Moreover, immune serum protected vaccinated mice even if these mice did not make antibodies on their own. This suggested that something else  besides antibodies [generated during vaccination] needed alongside with anti-viral antibodies for virus protection. 

Indeed, vaccinated mice depleted of anti-viral memory CD4 T cells just prior to WT virus challenge were not protected.

Next, the authors found that presence of anti-viral CD4 T cells (but not of irrelevant CD4 T cells) were necessary for anti-viral antibodies to enter and accumulate within infected neuronal tissue.

Finally, the authors showed that neuronal tissue recruitment of anti-viral CD4 T cells depended on α4β1 (VLA4 integrin) interactions and subsequent CD4 T cell-derived IFN-γ secretion mediated local vascular permeability to enable antibody access to neuronal tissue.

In summary, this study explains why simple application of immune serum is not always sufficient for protection against neurotropic infections. Neutralizing immune serum on its own is not able to penetrate barrier, immune privileged tissues in infected recipients if they lack anti-viral CD4 T cells. This could explain why anti-Ebola serum was not effective in all Ebola-infected patients. This study also provides mechanistic explanation for current paradigm for rabies virus protection that requires application of both vaccine [to induce T cells] alongside anti-rabies immune serum.

David Usharauli

Inflammatory signals from the peripheral tissue modulates immune response in lymph nodes

Very unusual results have been published in journal Science. This study came from Facundo Batista's lab at the London Research Institute.

The authors studied lymph node's adaptive re-organization in response to peripheral inflammatory signals, with the focus on CD169-positive macrophages. These specialized macrophages are ordinarily positioning themselves in the subcapsular sinus of the lymph nodes intercepting any incoming antigens from the peripheral tissue for presentation to B cells.

Using different type of inflammatory signals (live or dead bacteria, live or inactivated viruses, or TLR agonists), the authors observed that live bacteria or virus or TLR agonist injected in the mouse footpads caused temporal disruption of CD169⁺ macrophage layer in the draining lymph node.   

Further experiments showed that disruption of CD169⁺ macrophage layer in the draining lymph node upon CpG injection was abolished in mice selectively deficient for MyD88 signaling in dendritic cells.

In addition the authors noticed that disruption of CD169⁺ macrophage layer was also reduced in CCR7-KO mice where DCs could not migrate from the peripheral tissue to the lymph node.  

To understand physiological consequences of CD169⁺ macrophage layer disruption, the authors injected labeled B cell antigen into footpad following initial CpG or PBS (control) injection (+ 4 / +7 days later). These experiments revealed that fewer CD169⁺ macrophage and fewer antigen-specific B cells could acquire antigen following CpG injection.   

More importantly, the authors showed that initial CpG injection reduced subsequent anti-viral antibody response.

In summary, these results shows that initial inflammatory signals coming from peripheral tissue disrupt CD169⁺ macrophage layer and antibody response to subsequently injected antigen. 

This is strange results. No good explanation is given. For one, the authors did not provide results regarding viral titre or tissue pathology in mice injected with primary CpG. 2nd, isn't lymph nodes function to respond to the inflammation in peripheral tissue? How could it be beneficial for the host to shut down immune response following inflammatory stimuli? Could it be that lymph nodes are modified to activate different type of immune response (CTL?)

David Usharauli