Phage bet-hedging by replication slippage, and EBV antibodies
Slippage is not a replication defect but a phage hedge: a minority of individuals in the population shift reading frame through repeat-sequence changes, generating variants that have not yet been selected. Most fit the current environment; when bacterial defenses upgrade, the low-frequency alternatives take over immediately.
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What replication slippage leaves behind is standing variation
Phages do not just wait for random mutation. Their genomes contain "simple sequence repeats"; during DNA replication the polymerase occasionally slips, adding or deleting one or two bases, shifting the reading frame and toggling a specific protein between on and off. Unlike a frameshift at the translation stage, this is a permanent change at the DNA level and is inherited by progeny. Jolene's words: getting the polymerase to slip here or there is far easier and faster than acquiring a new gene or repairing a broken one.
— Jolene RamseyLow-level variants are a bet on the future
"Bet-hedging" means: most sequences fit the current environment, but a minority of low-frequency variants may happen to fit the next one. In the arms race between bacteria and phages, rather than waiting for a random mutation to be selected, it is better to let slippage keep producing low-frequency switches. Jolene compressed the logic into one line — you always have a set of low-level variants present, and one of them may be better suited to the next environment.
— Jolene RamseyAdenine repeat length decides which defense is switched on
The AGT gene itself trades off between two risks: the alpha-glucosyltransferase it encodes modifies phage DNA so it can evade the bacterial type 4 restriction-modification system, but the modified DNA is then easily recognized by other defense mechanisms. A run of adenine repeats in the gene comes in three lengths — 6, 7 and 8 — and different lengths give different reading frames, so protein function is either on or off. A single slippage event lets the phage shift rapidly between the two classes of defense without maintaining two sets of genes.
— Jolene RamseyReversion frequency is about ten thousand times higher than point mutation
They chose T4's R2A gene to test this: it has a run of 6 adenines, and the phenotype is trackable — on a host expressing the Rx defense system, once R2 function is lost, plaques cannot form. The experiments showed that the reversion mutation frequency in this SSR region is about ten thousand times higher than for ordinary point mutations. In other words, the phage can both switch this gene off by slippage to evade the current defense and rapidly switch it back on by the same mechanism.
— Jolene RamseyThe best-matched antibody has no in vitro shortcut
The study wanted human monoclonal antibodies that neutralize EBV, yet instead of relying only on an in vitro library such as phage display, it went back to hybridomas: fusing B cells from immunized mice with myeloma cells, selecting with HAT medium, and screening for hits on 384-well plates. The reason is that the somatic hypermutation and junctional diversity that occur as B cells mature add or delete bases in CDR3, which is hard to mimic in vitro. To get a perfectly matched antibody, you first let the B cell run the complete germinal center reaction inside the mouse, then take the heavy- and light-chain genes back into cultured cells for re-expression.
GP42 antibodies only neutralize B cell infection
All 8 antibodies against GP42 came from two clonal lineages, and the neutralizing mechanism is blocking the binding of EBV's GP42 protein to HLA class II; they therefore only block B cell infection and are ineffective against epithelial cells, because epithelium uses a different entry mechanism. The strongest of the 8 is ATX42-2. In subsequent humanized mouse experiments, with 500 micrograms injected intraperitoneally and 11 weeks of observation, ATX42-2 prevented tumor formation, viral DNA and splenomegaly.
Vaccine design is starting to target the key epitope
A GP350 subunit plus aluminum adjuvant did not induce protective immunity, and GP42, because of its conformational flexibility, is hard to raise the right antibodies against by conventional immunization. This paper's contribution is to define the key epitope and structural target when GP42 binds HLA class II, letting vaccine design be done by reverse engineering. Downstream, a GP350 ferritin nanoparticle vaccine has already shown safety and immunogenicity in phase 1, with seronegative participants followed to 540 days without infection; Moderna is meanwhile working on an mRNA vaccine containing gHgL, GP42 and GP220.
Anti-science may be the brain saving energy
Vincent used Kahneman's "Thinking, Fast and Slow" to explain anti-science: the brain defaults to low-energy Type 1 first, while deep verification belongs to Type 2, which requires extra energy and a startup cost. The Joshua Reynolds line on the wall — that people will do anything to avoid the labor of real thought — became the footnote to this observation. Someone without a statistics background who hears "Tylenol causes autism" will let lazy Type 1 simply accept it rather than go find the literature themselves. He stressed that this is only speculation, not a causal conclusion.
— VincentIn their own words · checked verbatim
So that you have a low-level variation always there that could be the better fit for the next environment.
Jolene Ramsey14:36
And just slipping the DNA polymerase here and there is way easier and faster than having to acquire a new gene or fix a gene that is broken.
Jolene Ramsey32:35
It's the fact that you're getting a full sort of diversification process in the mouse.
There is no expedient to which a man will not resort to avoid the real labor of thinking.
Vincent1:42:39
That's lazy type one thinking instead of going to the literature and reading it yourself.
Vincent1:46:40
Figures
| Adenine repeat lengths in the AGT gene | 6, 7, 8 | 18:48 |
| Difference between SSR reversion frequency and point mutation | 10,000-fold | 20:51 |
| Number of SSRs in phage T7 | 0 | 26:01 |
| Number of GP350-binding antibodies | 2 | 54:55 |
| Number of GP42-neutralizing antibodies | 8 | 54:55 |
| Antibody injection dose | 500 micrograms | 1:04:26 |
| Weeks of observation | 11 weeks | 1:04:26 |
Glossary
- simple sequence repeats (SSR)
- Tandem repeats of short motifs in a genome; polymerase slippage during replication changes the repeat number and toggles gene function.
- bet-hedging
- Long-term retention within a population of a minority of variants that do not fit the current environment but may fit a future one, as a response to unpredictable change.
- hybridoma
- An immortalized cell line made by fusing an immunized B cell with a myeloma cell, capable of continuously secreting a single antibody.
- HAT medium
- A selective medium containing hypoxanthine, aminopterin and thymidine, used to select successfully fused hybridoma cells.
- humanized mice
- Mice engrafted with human hematopoietic stem cells or immune cells, used to partially reconstitute human immune processes in vivo.
How to listen
R&D people and biotech investors working on vaccines, monoclonal antibodies or phage therapy, who want to understand mechanisms rather than just read pipeline news.