A very interesting and thought-provoking research in humoral immunity was published in journal Science this week. It came from Bruce Beutler's lab at the UT Southwestern Medical Center. Professor Beutler, as many may know, is a 2011 recipient of Nobel Prize in Physiology or Medicine for his discovery of endotoxin receptor (TLR4).
Actually, before I review this paper I would like to highlight that since 2005 when Ruslan Medzhitov's lab has published the first paper in Nature that suggested the role of TLR in B cell responses and then later in 2006, David Nemazee's lab has published new study in Science questioning the validity of 2005 paper, there was a controversy in this field. In fact, in 2009, Ruslan Medzhitov's lab has published another study that confirmed that haptenated proteins used in 2006 study could promote antibody response independent of TLR signaling. However the mechanism remained elusive.
It appears that this new study in Science may be a first glimpse in the darkness to reconcile the differences.
I also would like to point out that Professor Beutler's lab is famous for studying ENU-induced random mutations in mice to generate unbiased molecular signature of immune deficiencies.
Humoral immunity is divided into T-dependent and T-independent (TI) antibody response. TI responses, are in turn segregated into two categories: TI-1 and TI-2. This new study focus on TI-2 response.
Initially, using NP-Ficoll immunization, as a TI-2 model antigen, the authors observed that NP-specific IgM response was diminished in single STING-KO, cGAS-KO, MAVS-KO mice but not in several TLR signaling KO mice.
Mice, double deficient in cGAS/MAVS, had almost complete absence of NP-specific IgM response.
This was strange observation since STING, cGAS and MAVS are known to detect the presence of DNAs or RNAs and NP-Ficoll contained none of them.
Using adoptive B cell transfer from these three KO mice into RAG-KO hosts, the authors showed that this effect of NP-Ficoll was B cell intrinsic (and the number of B1 or MZ B cells were normal in non-responding KOs).
The authors concluded that DNAs and RNAs were generated as a result of B cell stimulation by NP-Ficoll. Indeed, several endogenous retroviruses were up-regulated in NP+ B cells as compared to NP- B cells.
Furthermore, NP-ficoll immunization induced Reverse Transcriptase (RT) activity in NP+ B cells.
Parallel experiments revealed that mice deficient in NF-kB signaling also lacked the ability to respond to NP-Ficoll immunization or express endogenous retroviruses.
The authors observed that B cells from MAVS-KO, but not from cGAS KO mice, showed reduced phosphorylation of NF-kB proteins after anti-IgM stimulation, implying that MAVS played more critical role in sustained activation of NF-kB.
In summary, the authors proposed the new model of TI-2 antibody response. According to this model, when antigens with repetitive epitopes engage specific B cells, this activates initial wave of NF-kB activity leading to expression of endogenous retroviruses as a RNA that are subsequently converted into cDNA by RT activity. These newly generated RNAs and cDNAs are recognized by MAVS and cGAS/STING pathways, respectively. In turn, MAVS activation induces second wave of NF-kB activation to sustain TI-2 antibody response.
As the authors correctly pointed out, at this stage, it is not clear whether expression of endogenous retroviruses in activated B cells indeed play a critical role in TI-2 response. Their presence could be coincidental to this process. It would require B cell devoid of endogenous retroviruses to definitely test their precise role (since both TI-1 and T-dependent antibody response were shown to be STING, MAVS and cGAS independent).
For me this results represent new concept how we should assess the role of endogenous viruses present among our genes. Not everyone carries them, even among laboratory mice strains there is significant differences. Why would nature develop such system where TI-2 response would depend on endogenous viruses? It is just a fascinating idea.