Therapeutic HSV-2 Vaccine: Critical Goal or Pipe Dream?
To those who have followed my blog in the past, please accept my apology for about 7 months of radio silence. I took a Sabbatical Leave at the Rocky Mountain Laboratories from Dec 2013 to June 2014 to brush up on my techniques, and develop some new assays to measure the T-cell response to a live-attenuated HSV-2 vaccine. Dr. Kim Hasenkrug and the great people in his lab made this a fabulous 7 months. I learned a ton, and this was time well spent. I am a skier, and I can also report that Lost Trail ski area (http://www.losttrail.com/), which is 45 miles south of the lab, gets very high marks for tons of fresh powder in early 2014!
The last 2 months of my Sabbatical were ridiculously busy, and the 6 months since I returned to my home institution at SIU School of Medicine have been equally nuts. Very productive, but no time for a life and no time for the blog….hence the long silence.
So, without any more excuses, time to get down to business. I said in May 2014 that I would write three posts to address a reader’s questions, which went something like this: “Let’s discuss the therapeutic effect of a future vaccine. So basically the patient gets a vaccine shot in his arm, just like all other vaccines are given…How would an arm shot benefit the genital area for protective or therapeutic purpose?”
There are three components to this question, and these are:
1) How does the human immune system respond to a HSV-2 vaccine?
2) Would a shot of HSV-2 vaccine in the arm be enough or would a shot in the behind provide better protection against genital herpes?
3) What is the goal of a ‘preventative’ versus a ‘therapeutic’ HSV-2 vaccine, and are each of these goals equally feasible?”
I addressed the first two questions in earlier blog posts:
I address the third and final question below in this blog post. Way to be a slacker Halford…..it only took you 7 months!! Well, better late than never…here it goes.
1) PREVENTATIVE HSV-2 VACCINES.
The basic principles on which the natural world and biology operate are generally pretty straightforward. That said, human beings have an infinite capacity to screw things up and make something that should be simple seem far more complex than necessary. This is the situation with HSV-2 vaccines…..what should be a simple problem has become a quagmire of contradictory logic and bad ideas.
A preventative HSV-2 vaccine would be simple if we considered a live-attenuated HSV-2 virus as the means of vaccination. Worked for smallpox, measles, chickenpox, etc and would work for HSV-2, but we have not tested any live HSV-2 vaccines in the U.S. So where things have gotten complicated is that scientists hypothesized that a live-attenuated HSV-2 vaccine might be “too dangerous” to pursue (based on no compelling evidence to support the claim), and scientists started developing “safer alternatives.” The need for these “safer alternatives” has never been proven, but nonetheless HSV-2 vaccine researchers have spent >99% of their time and money studying vaccine approaches based on just a few bits or pieces (subunits) of HSV-2.
When I say subunit vaccines, I am specifically talking about approaches like…….(1) the Agenus HerpV vaccine, (2) the Genocea GEN-003 vaccine (i.e., Herpevac + ½ of another HSV-2 protein), (3) Vical’s HSV-2 vaccine, (4) the Corridon / Admedus HSV-2 vaccine fundraising effort. This general subunit vaccine concept has been failing in human clinical trials since the mid-1980s, and there is no compelling evidence that the newer versions of this old concept will be any better as preventative HSV-2 vaccines.
If one INCLUDES a live-attenuated HSV-2 vaccine as an option in our human clinical testing of HSV-2 vaccine candidates, then a preventative HSV-2 vaccine should be a no-brainer provided that (1) the live HSV-2 vaccine in question elicits at least 30% of the pan-HSV-2 IgG antibody response that is observed in asymptomatic carriers of wild-type HSV-2 and (2) this vaccine-induced HSV-2 antibody response is durable and is maintained at a detectable level for at least 3 years post-vaccination.
In contrast, if one EXCLUDES a live-attenuated HSV-2 vaccine as an option for human clinical testing of HSV-2 vaccine candidates (which has been the case in the U.S. for the past 30 years), well that would make it damned near impossible to develop an effective HSV-2 vaccine.
So, a preventative HSV-2 vaccine is easy if one uses the right tool that nature has provided us for the job (a live-attenuated vaccine), but human beings have complicated the issue by creating a scientific culture is which most ‘experts’ have arrived at precisely the wrong conclusion that “We should test any and all HSV-2 vaccine concepts except for the ‘most dangerous approach’ of a live-attenuated HSV-2 vaccine.”
Call me crazy, but I am going to go out on a limb and suggest that human testing of a well-vetted live-attenuated HSV-2 vaccine in n=20 people is probably much safer than what we have been doing for the past 30 years……watching n=10 to 20 million people per year get newly infected with HSV-2 while scientists keep tinkering with HSV-2 subunit vaccines that don’t work.
2) THERAPEUTIC HSV-2 VACCINE: CRITICAL GOAL OR PIPE DREAM?
Everything I write above may be distilled into one sentence: The likelihood that a live-attenuated HSV-2 virus would function as a very effective, preventative vaccine to prevent HSV-2 genital herpes is similar to the probability that the sun will rise tomorrow.
I will be the first to admit that I don’t have a crystal ball, and I really cannot see into the future. However, in both cases, experience tells me that it would be prudent to assume these two things are very, very, very likely to come to pass, and thus we should probably plan accordingly. I have no plans to hang myself or shoot myself tonight on the off-chance that the sun won’t rise tomorrow, and the world will suddenly become an uninhabitable rock. Likewise, there is no basis in logic or reason to assume that a live-attenuated HSV-2 vaccine should be any less likely to succeed than the live-attenuated viruses that have been successfully used to control smallpox, yellow fever, polio, mumps, measles, rubella, and chickenpox.
As certain as I am that a PREVENTATIVE HSV-2 VACCINE is infinitely feasible, I would put myself somewhere precisely in the middle when it comes to the feasibility of a THERAPEUTIC HSV-2 VACCINE.
Before proceeding, I offer my simple take-home message on therapeutic HSV-2 vaccines in the next two paragraphs.
The promise of a therapeutic HSV-2 vaccine lies in that, in theory, it might be (1) administered to a person with uncontrolled HSV-2 genital herpes outbreaks and (2) might re-program that person’s adaptive immune system to do a better job in the future of controlling their HSV-2 infection such that their outbreaks either stopped altogether, or occurred at a much lower frequency. The importance of this possibility for tens of millions of herpes sufferers cannot be overstated, and thus the possibility of a therapeutic HSV-2 vaccine deserves the scientific community’s full attention.
Despite the importance of this possibility, it is equally important for HSV-2 genital herpes sufferers to recognize that, unlike a preventative HSV-2 vaccine, there is simply no historical precedent that dictates that a therapeutic HSV-2 vaccine MUST be possible. A HSV-2 therapeutic vaccine is an important possibility, but it remains to be determined if the idea falls into the realm of “science” or “science fiction.” Maybe someone who has lived with symptoms of recurrent HSV-2 genital herpes that have been ongoing for years may be “dialed back” to zero symptoms, or close to zero symptoms, by immunizing them with a series of “therapeutic HSV-2 vaccine shots.” It is an appealing idea (or hypothesis), but it is a hypothesis that remains largely untested, and thus unknown.
Below I consider some of the theory that underlies (1) the counterargument why a therapeutic HSV-2 vaccine SHOULD NOT be possible and (2) the argument why a therapeutic HSV-2 vaccine MAY BE possible.
ARGUMENT AGAINST A THERAPEUTIC HSV-2 VACCINE
The argument against a therapeutic HSV-2 vaccines that immunologists (including myself) have been trained to think in terms of is that…….”immunity to HSV-2″ is a binary variable (i.e., a 0 or a 1)……you either have it or you do not.
With this rationale in mind, the story goes something like this………….
After a person is infected with HSV-2, their body’s B- and T-lymphocytes mount a response to the HSV-2 virus. The activated B-lymphocytes produce HSV-2-specific antibodies that circulate through the blood and lymph. The activated T-lymphocytes enter the circulation and migrate into HSV-2-infected tissues. Collectively, the development of (1) HSV-2-specific antibodies and (2) HSV-2-specific T-cells constitutes “immunity to HSV-2,” and under this logic, a therapeutic HSV-2 vaccine should NOT be possible.
That is, if being infected with wild-type HSV-2 is sufficient, in and of itself, for a person to convert from being “naïve” (i.e., a 0) to “HSV-2-immune” (i.e., a 1), then how could a therapeutic HSV-2 vaccine possibly help? Is the therapeutic HSV-2 vaccine going to make them a 1+?
There are at least two limitations in this logic.
First, in the natural world, complex biological processes like the development of an adaptive immune response to HSV-2 are rarely (if ever) describable in terms of a purely binary variable (i.e., black=0 or white=1). Hence, a more realistic (although still overly simplistic) way to think of “protective immunity to HSV-2″ is to think of this immunity as lying on a scale of 0.0% to 100.0% possible protection against HSV-2.
Second, if we consider the actual basis of immunity to HSV-2, this relies on a diverse population of millions of B-lymphocytes, CD4 T-helper lymphocytes, CD8-lymphocytes, and likely some less studied CD4+CD25+ regulatory T-lymphocytes (which can dampen a protective immune response, and push the immune system towards “tolerance”). Aside from the sheer numbers of cells involved, these lymphocyte may recognize hundreds to thousands of different bits (epitopes) of HSV-2 distributed across 75 viral proteins. To help put this in perspective, it is useful to remember that a single B-cell or T-cell recognizes a very small piece of HSV-2 on the order of 6 to 25 amino-acids in length. In total, HSV-2 encodes 39,100 amino acids worth of protein. Given the reality of how the biological system operates, it is really not accurate to suggest that “immunity to HSV-2″ may be described as a binary variable, and thus to think of human beings as either being HSV-2 seronegative (naïve) or HSV-2 seropositive (immune). Such descriptions are such gross oversimplifications of the truth that they are about 10% right and are about 90% wrong. Nonetheless, that is how most doctors and scientists have been trained to think of immunity to HSV-2 (+/-). This mindset (1) creates the illusion of understanding for doctors; (2) is based on ignorance of how the adaptive immune response to HSV-2 actually operates; and (3) creates no opportunity for people to rationally discuss and consider the possibility of a therapeutic HSV-2 vaccine, which requires a more accurate and nuanced understanding of HSV-2 immunity.
So, we return to the primary counterargument against a therapeutic HSV-2 vaccine. The suggestion that prevents most doctors and scientists from considering a therapeutic HSV-2 vaccine is that we have been trained to believe that infection with wild-type HSV-2 renders a person fully immune to HSV-2. I offer three scenarios below that suggest it is vanishingly unlikely that everyone who is infected with HSV-2 immediately acquires 100% of the protective immunity against HSV-2 that is possible.
1) 80% of people who are infected with HSV-2 mount an antibody response to the virus, but never experience any symptoms. Thus, it is a well known fact that 80% of HSV-2 infections are completely asymptomatic, and it would appear that in these individuals that the protective immune response might approach 100% of the protective immunity against HSV-2 that is possible.
2) People who develop symptomatic HSV-2 herpes often take 4 to 12 months to effectively control the primary infection. That is, they have HSV-2 outbreaks almost continually for the first 4 to 12 months of the infection. Likewise, these same individuals may not “seroconvert” and become clearly positive for HSV-2 antibodies in their blood until 4 months after the initial infection. In contrast, animals that are immunized with wild-type HSV-2 or a live-attenuated HSV-2 vaccine mount a very potent antibody response within the first 30 days after immunization. If the people whose primary HSV-2 infection (1) drags on for 4 to 12 months and (2) it takes 4 months for their antibody levels to HSV-2 to become detectable, is it really reasonable to assume that this represents “100% of the protective immunity to HSV-2″ that is possible?
3) A subset of HSV-2 infected people (~1 per 1,000 infected persons) develop recurrent HSV-2 genital herpes with outbreaks often occurring 6 to 12 times per year, and sometimes more often. This pattern may continue for over 20 years. Is this level of immune control of a HSV-2 infection really the best the adaptive immune system has to offer? It seems possible to me that some, or perhaps all, of individuals who suffer with HSV-2 genital herpes may be stuck in a vicious cycle where their adaptive immune system is just not learning from their repeated encounters with HSV-2. If this is the case, then it is possible that some HSV-2 genital herpes sufferers may only possess 1 to 10% of the “protective immunity to HSV-2″ that is possible. If this were indeed the case, then it might explain why some people experience >4 outbreaks per year and why some have outbreaks that last on the order of 10 to 21 days at a time.
None of the points I bring up above prove that a therapeutic HSV-2 vaccine is possible. However, I note that there is no hard evidence that supports the prevailing belief that recurrent HSV-2 genital herpes “just happens” in the face of “100% of the protective immunity to HSV-2″ that is possible. I would suggest that if a person’s immunity to HSV-2 was really all that, then logically they should either be (1) asymptomatic or (2) should only get very occasional and very brief outbreaks of HSV-2 genital herpes. It is hard to reconcile how someone may possess “100% of the protective immunity to HSV-2″ that is possible, and yet still experience 4 or more recurrences of HSV-2 genital herpes each year where each outbreak may last for 7 to 21 days. Sounds like pretty lousy “immunity” to me.
WHY MIGHT A THERAPEUTIC HSV-2 VACCINE BE POSSIBLE?
If we are willing to consider the possibility that a subset of people who live with >4 outbreaks per year of HSV-2 genital herpes may possess less than 100% of the attainable protective immunity to HSV-2, then this leaves room for improvement (or re-programming) of an individual’s adaptive immune response to HSV-2 via a therapeutic HSV-2 vaccine. Again, I ask the reader to please bear in mind that everything that follows is simply theoretical (speculative), and is nothing more than a tour of the hypothetical. Therefore, the reader should not arrive at the end of this post with the certainty that a therapeutic HSV-2 vaccine must be feasible. No such certainty exists. Rather, what I outline below simply represents a possibility that merits more careful examination by the scientific community.
1. Therapeutic HSV-2 vaccines in current clinical trials.
I note that several therapeutic HSV-2 vaccines are being tested in human clinical trials (e.g., GEN-003, HerpV), but I think these approaches represent pretty lame ideas. The HerpV vaccine is a T-cell only vaccine. Newsflash…..antibodies are important too. A “T-cell only” vaccine is about as good as the left half of a condom or half of a bicycle helmet. Call me a perfectionist, but I generally find that half-protection is little more than the illusion of protection with all of the dangers of no protection at all. GEN-003 is just Herpevac-plus, and the Herpevac-approach has already failed in six human clinical trials. So, brace yourself, but it is just a matter of time until GEN-003 (Herpevac-plus) is shown to be as lame as all of the Herpevac-like predecessors that came before it. Tweaking the same bad idea (gD-subunit) by adding half a protein (half of ICP4) and changing the name to “GEN-003,” does not alter the fact that it is still an approach that is eerily similar to a bad idea that failed in human clinical trials in 1990, 1994, 1997, 1999, 2002, and 2012.
When I talk below about the “potential of a therapeutic HSV-2 vaccine to reduce herpes symptoms,” I am talking about a real, honest-to-God live HSV-2 vaccine that is 99% similar to wild-type HSV-2. I am not talking about GEN-003, HerpV, Vical, or Admedus or whatever permutation of the subunit vaccine approach (based on <3% of HSV-2’s proteome) is supposedly going to elicit 100% of the protective immunity against HSV-2 that is possible. Just like it is a bad idea to enter a boxing ring with both of your arms tied behind your back, I think it is a bad idea to show the human body a wimpy “<3% of HSV-2″ subunit vaccine and hope it will elicit 100% of the protective immunity against HSV-2 that is possible.
2. Therapeutic HSV-2 vaccines that might have a better chance of working
Even with the best possible HSV-2 vaccine imaginable, a therapeutic HSV-2 vaccine that prevents or reduces the symptoms of HSV-2 genital herpes is going to be a heavy lift. Thus, no matter how well a therapeutic HSV-2 vaccine is designed, there can be no assurances that it will work.
If one wanted to stack the deck in favor of a therapeutic HSV-2 vaccine, a few of the essential features of the best possible therapeutic HSV-2 vaccine would be that:
(1) It would be a good B-cell antigen that engages the broadest repertoire of HSV-2-specific B-cells found in the body;
(2) It would be a good T-cell immunogen that engages the broadest repertoire of HSV-2-specific T-cells found in the body;
(3) It would achieve the broadest possible breadth of B- and T-cell stimulation possible by carrying or encoding >95% of HSV-2’s antigens; and
(4) All of these properties defined above would elicit the broadest possible HSV-2-specific antibody response, HSV-2-specific CD4+ T-cell response, and HSV-2-specific CD8+ T-cell response.
In light of these desired properties of a therapeutic HSV-2 vaccine, I would suggest that my laboratory’s live-attenuated HSV-2 0DNLS vaccine strain would be a far better candidate for a therapeutic HSV-2 vaccine because (1) it encodes 99.3% of HSV-2’s foreign proteins and (2) it establishes a short-lived infection (~3 days) at the site of immunization. While some might claim a live-attenuated HSV-2 vaccine would be “too dangerous,” I would remind everyone that this attenuated virus is ~10,000-fold less virulent than the wild-type HSV-2 that herpes sufferers already carry, so the “too dangerous” argument is largely irrelevant in the context of a therapeutic HSV-2 vaccine.
3. How would a therapeutic HSV-2 vaccine work?
Assuming one starts with a reasonably designed therapeutic HSV-2 vaccine, below I outline how it might work to reduce the symptoms of HSV-2 genital herpes.
The immune system must, at all times, make a decision about what substances it deems “foreign” and decides to attack versus substances that it deems “self” (or close enough to self) that it decides it should not attack. In the context of HSV-2, I am sure many are thinking, “What is the point of this decision….attack that little son of a bitch that is HSV-2!!!” Well, the downside of having an overzealous immune response that spills over from things that are foreign to things that are you (self) is called “autoimmune disease.” Examples include lupus, Sjögren’s syndrome, and Grave’s disease…..very real problems that our bodies try to avoid by making very judicious decisions about what precisely the immune system chooses to attack.
In the case of chronic infections, like HSV-2 genital herpes, it seems that these infectious agents have developed an array of tricks to fool the immune system into becoming tolerant to many of their foreign proteins (i.e., tricking the immune system into seeing a microbial protein as “self” that should not be attacked). So, even though your bone marrow makes a truckload of HSV-2-specific B- and T-cells, it is unclear, even in people who have been infected with HSV-2 for years to decades, that the full repertoire of HSV-2-specific B- and T-cells (and the antibodies made by B-cells) have been fully enlisted to help combat these infections. Thus, it is possible that someone with HSV-2 recurrent genital herpes may only possess 1 to 10% of the protective immunity against HSV-2 genital herpes that is possible, because most of your B- and T-cells have been tricked into seeing HSV-2 as self and entering a state of anergy (i.e., non-responsiveness).
So, finally….after explaining a lot of immunology background, we come to the reason that a therapeutic HSV-2 vaccine might work. Wild-type HSV-2 can slink around in your body and hide relatively well from detection by the host immune system…..this is a known quantity. HSV-2 encodes at least 4 proteins that trick the immune system and delay recognition of the infection; namely, ICP47 which delays CD8 T-cell recognition, gE-gI, which dampens the useful effects of antibody; gC, which dampens the activation of the complement cascade; and ICP34.5, which antagonizes PKR / interferon-induced shutoff of viral protein translation (particularly in neurons). So, it should not be news to anyone that HSV-2 messes with the immune system pretty hard. The new wrinkle I am suggesting here is that it is possible (but absolutely unproven) that as small quantities of HSV-2 (100s to 1000s of infectious virus) are slinking around a person’s body and delaying recognition and control by the host immune defenses, this process could be the very thing that tricks their B- and T-cells into becoming complacent (anergic) and not responding efficiently to suppress HSV-2 reactivation events as soon as they flare up. So, instead of having a 2-day subclinical outbreak that is controlled before you ever see a lesion, because the adaptive immune response to HSV-2 is on permanent holiday (i.e., a weak antibody response and an inadequate T-cell response), HSV-2 reactivation events are allowed to slink along and cause 2 to 3 weeks of disease in a subset of ~1 per 1,000 HSV-2 infected persons who, for whatever reason, have simply failed to stitch together 100% of the protective immunity against HSV-2 that is possible.
If this hypothetical scenario is true, then a live-attenuated HSV-2 vaccine could work as a therapeutic HSV-2 vaccine by delivering a 5,000- to 10,000-fold higher dose of a live-attenuated HSV-2 virus into the epithelium (skin) than a herpes sufferer would ever produce during an outbreak. That live-attenuated virus could replicate and spread in the skin around the injection site for 2 to 4 days, and teach (re-program) the immune system to better recognize HSV-2’s foreign proteins.
Let us hypothetically say that the dose of live-attenuated HSV-2 vaccine delivered in the skin was 40 million infectious units. That would represent about a 20-fold increase relative to what I have given 30 g mice. A relatively small human weighs at least 40,000 g, and so on a mg / kg basis, 40 million infectious units of a live-attenuated HSV-2 vaccine would be a reasonable guesstimate of how to appropriately scale a live-attenuated HSV-2 vaccination of mice up to a scale appropriate for an animal the size of a human.
Unlike wild-type HSV-2 which is good at hiding its protein antigens and flying below the radar of the immune system as it slinks along based on a few thousand infectious units of virus (i.e., smaller than the population of South Lake Tahoe), a single shot of a therapeutic HSV-2 vaccine containing 40 million infectious units of a live-attenuated virus would be like introducing the immune system to the entire population of California. Such a large dose of infectious virus negates any possibility that HSV-2, however devious and clever, may hide its proteins from the immune system when delivered in such a large bolus dose. This would introduce the possibility of bringing together all of the necessary ingredients to elicit a far more protective immune response against HSV-2; namely, (1) inflammation, (2) appropriate danger signals, and (3) nearly all 75 of HSV-2’s foreign proteins being expressed in their native context of (i) virus-infected cells that serve as HSV-2 virion factories and (ii) the infectious HSV-2 virions that are released from those virus-infected cells.
Whether or not 2 or 3 shots of such a therapeutic HSV-2 vaccine would be sufficient to alter the duration or frequency of HSV-2 genital herpes outbreaks in sufferers remains to be determined. However, I believe that it is an obvious enough possibility that it deserves the scientific community’s attention.
We could try and achieve the potential benefits of a therapeutic HSV-2 vaccine with the Agenus HerpV vaccine (T-cell only vaccine), Genocea GEN-003 (<2% of HSV-2), or U.V.-inactivated HSV-2 (not a live virus) as was advocated by Dr. Gordon Skinner (http://www.ncbi.nlm.nih.gov/pubmed/11063759). I think there are a lot of simple and valid immunological reasons that each of the these approaches should be less effective than a live-attenuated HSV-2 vaccine, and all of those detailed immunological considerations effectively reduce to the same reason that one does not enter a boxing ring with their arms tied behind their back……most of us don’t like getting our butts kicked the way HSV-2 vaccines based on <3% of HSV-2’s proteome have been getting their asses handed to them for the past 30 years.
If the world wishes to test a therapeutic HSV-2 vaccine with any hope of success, I would suggest that a live, replication-competent, but appropriately attenuated HSV-2 virus would be the right tool for the job, and my lab’s live-attenuated HSV-2 ICP0- mutant viruses represent just one of many possible approaches that could be taken (http://www.ncbi.nlm.nih.gov/pubmed/20808928).
Whether or not a therapeutic HSV-2 vaccine is even possible remains an open question. If my laboratory is able to acquire the financial means to test such a concept with our live HSV-2 ICP0- mutant viruses, then I would enlist the help of herpes sufferers around the world to volunteer for such clinical trials.
If you have HSV-2 genital herpes and would like to be added to such a list, in the event a therapeutic HSV-2 vaccine trial of my lab’s live-attenuated HSV-2 vaccine comes to pass, then please let me know by e-mailing [email protected]
As I said at the beginning of this post….. I meant to write this blog post back in June 2014…. better late than never.
– Bill H.