B cell intrinsic TBK1 is essential for germinal center formation
during infection and vaccination in mice

Research Background
Current COVID-19 pandemic has brought into our attention that there are several unknown
evasion mechanisms by pathogens. To protect host from infection/re-infection, immunologicalmemory must be elevated via one time infection or vaccination. Although this is the main study
area of immunology field, however, little is known how immune memory is developed and how
long it is durable during infection and vaccination. As an example, malaria infection is one of themost troublesome infections in the world and known that an individual remains susceptible torepeated malaria infection. To date, how malaria infection impairs immune memory that confers
sterile protection is not fully understood.
Germinal centers (GC) are highly dynamic structures formed in lymphoid organs and are sites
where somatic hyper mutation and clonal selection are coupled for high-quality durable antibody
responses leading immune memory. The anatomical structure and function of GCs have beenstudied for a long, but only recently information has been gathered about how B cells are guided
through the development of GCs. Upon antigen encounter, B cells rapidly proliferate anddifferentiate into multipotent precursor cells that have the potential to diverge into two distinct
fates: either GC or extra follicular (EF) responses. Induction of GC reaction is often the desirable
outcome of an infection or vaccine to provide antigen-specific long-lasting humoral immunity.
Conversely, the induction of GC-independent EF responses in several disease models andvaccinations results in early acute production of short-lived plasma blasts. The intrinsic andextrinsic factors that determine the fate of antigen-activated B cells, whether to enter GC reaction
or to commit to EF responses are not yet well-defined.
TANK-binding kinase 1 (TBK1) is a serine/threonine kinase that is famously known for its role ininnate immunity especially for type I interferon (IFN-I) production upon nucleic acid recognition.
Apart from its role for IFN-I production via phosphorylation of IRF3/7, TBK1 is also involved invarious signaling pathways in a cell-type specific manner. Earlier studies found that TBK1 isamong one of the genes upregulated in GC B cells upon NP-CGG immunization, suggesting that1

TBK1 may have a role in GC formation. However, whether TBK1 plays a critical role in GCdependent memory responses have remained un-answered.

Outline of research results
Here, using both malaria infection and immunization models, we identified TBK1 as a novel B cell
intrinsic factor that is crucial in regulating GC differentiation. We found that phosphorylation ofTBK1 increased in B cells during GC differentiation (Figure). Notably, TBK1 deletion in B cells led
to impaired GC formation and therefore increased susceptibility to a non-lethal malaria infection.
Memory B cells generated in the absence of TBK1-dependent GC failed to confer sterile immunity
upon re-infection challenge. Although TBK1 deficient B cells could be activated to differentiate into
GC precursor cells (Pre-GC), the fate of these cells was distinct from that of TBK1 sufficient Bcells. We found that TBK1 negatively regulates both CD40 and BCR signaling in Pre-GC.
Mechanistically, increased CD40-induced RelB expression in Tbk1-deficient Pre-GC enhanced IRF4expression that subsequently suppressed Bcl6 expression, the master regulator of GC formation.
Unlike naïve B cells, BCR activation further enhanced CD40-induced IRF4 expression in Pre-GC ina TBK1-dependent manner. This study revealed a crucial role of B cell intrinsic TBK1 as adeterminant of GC commitment through the fine tuning of CD40 and BCR signaling (Figure).

Figure: Role of B cell intrinsic TBK1 for germinal
center formation in lymphoid organs
Wild type mice (top of the figure) and B cell intrinsic
TBK1-deficient mice (bottom of the figure) responded
differently to vaccination and malaria infection due todistinct B cell fate decision. The horizontal axis
represents the stages of germinal center B celldevelopment. The vertical axis represents the expression
level of B cell signaling molecules. B cell receptor (BCR)
and costimulatory receptor (CD40) engagement activates
CD40-BCR signaling to initiate B cell activation. P stands
for phosphorylation for TBK1 activation, GC stands forgerminal center.
2Future prospects
Overall, our findings are expected to have crucial implications for tailoring successful vaccines toproduce long lasting protective immunity against several problematic infections like malaria as
well as a countermeasure to future pandemics.
TBK1 is required for IFN-I mediated antiviral immunity, while here we additionally showed its
importance in GC-mediated humoral immunity. However, TBK1 is also involved in thepathogenesis of autoimmune diseases. Moreover, TBK1 inhibitor, such as Amlexanox has beenused in clinical treatment of asthma and recently been proposed for the treatment of obesity
related metabolic diseases. Our findings suggest that TBK1 inhibitors should be administered withcaution, considering its possible effect on humoral immunity in case of infection and vaccination.
Moreover, the timing of TBK1 inhibitor administration may be important to target TBK1 in specificcell types during disease progression and to prevent its suppressive effects on vaccine efficacy.

Journal: Journal of Experimental Medicine (JEM) online
Title: B cell intrinsic TBK1 is essential for germinal center formation during infection andvaccination in mice.
Authors: Michelle S. J. Lee, Takeshi Inoue, Wataru Ise, Julia Matsuo-Dapaah, James B. Wing,Burcu Temizoz, Kouji Kobiyama, Tomoya Hayashi, Ashwini Patil, Shimon Sakaguchi, A. Katharina
Simon, Jelena S. Bezbradica, Satoru Nagatoishi, Kouhei Tsumoto, Jun-Ichiro Inoue, Shizuo Akira,Tomohiro Kurosaki, Ken J. Ishii, Cevayir Coban＊ (*corresponding).

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