How does the immune system respond to a HSV-2 vaccine?

It has been over 6 months since I made a new post on the blog.  Sorry about the hiatus…..the laboratory has been occupying all of my time, but I managed to carve out some time to work on the following post.   Someone sent me a query / comment on this blog last month that I thought might be of general interest to many readers. The blog post below is my response.

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QUESTION:

Dear Dr. Bill,

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 does the human immune system respond next in the case of Acam-529 as an example?  Antibodies and T-Cells get generated. Would an arm shot be enough or yet better a shot in the behind area / under the belt? How would an arm shot benefit the genital area for protective or therapeutic purpose?

– Curing

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Dear Curing,

There are a few different questions here, and so I will break them apart and tackle them one by one in a series of blog posts. Specifically, your questions are “Let’s discuss the therapeutic effect of a future HSV-2 vaccine.

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? How would an arm shot benefit the genital area for protective or therapeutic purpose?”

3) In addition, I will add the following question into the mix here that I believe is relevant:  “What is the goal of a “preventative” versus a “therapeutic” HSV-2 vaccine, and are each of these goals equally feasible?”

 

For today, I focus on addressing the first question.

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HOW DOES THE HUMAN IMMUNE SYSTEM RESPOND TO A HSV-2 VACCINE?

At a minimum, there are two essential ingredients in any vaccine and these are (A) an irritant that establishes a pro-inflammatory response at the site of injection and (B) one or more foreign components that significantly differ from the molecular complexes and/or cells that are part of “self” (i.e., that which is normally present in the human body).

A. The first ingredient in any vaccine is an “irritant activity,” which is commonly introduced into a vaccine in the form of an adjuvant. The word adjuvant effectively means a “helper” or “facilitator.” The purpose of the irritant activity in a vaccine adjuvant is to help elicit “danger signals” (pathogen-associated molecular patterns) that activate cells of the immune system and induce them to turn on, or upregulate, molecules and machinery that render the body’s B- and T-lymphocytes competent to respond to the foreign components in a vaccine. Specific examples of the types of molecules that are up-regulated by the irritant activity in adjuvants include co-stimulatory molecules such as CD28, B7, CTLA4, and antigen-presentation molecules such as MHC class I and II.

The key concept here is that the optimal / most productive immune response to the foreign components of a viral vaccine cannot be mounted by any one cell of the immune system. Rather, productive immune responses to a viral vaccine are essentially a decision that will optimally involve a “committee” of at least five different cell types, and these cell types are: (1) virus-infected structural cells in the case of a live viral vaccine, such as ACAM-529 (e.g., epithelial cells of the skin); (2) professional antigen-presenting cells such as dendritic cells or macrophage; (3) B-lymphocytes (which give rise to antibody-producing cells); (4) CD4+ T-helper cells; and (5) CD8+ T-cells, which can directly interact with virus-infected cells by (a) killing virus-infected cells or (b) secreting cytokines that directly suppress virus replication.  The picture at the top of this post illustrates most of the members of the cellular committee that respond to a vaccine.

These five different populations of cells only become fully competent to “act as a committee” (talk to one another) in response to a viral vaccine if they receive adequate danger signals, and this is one of the major functions of adjuvants such as alum (aluminum salts) and/or monophosphoryl lipid A (MPL). This combination adjuvant is used in the Gardasil vaccine and was used in the Herpevac vaccine (i.e., a failed HSV-2 vaccine). So, when considering vaccines, it is critical that there is either an adjuvant or some other, comparable, pro-inflammatory activity.

A second, critical aspect of this pro-inflammatory activity is that it actively recruits immune cells (white blood cells) to the site where the vaccine is injected, and thus ensures that the foreign components in the vaccine are seen (encountered by) large numbers of white blood cells. In the case of HSV-2 ACAM-529, this is actually a genetically modified version of the HSV-2 virus, and thus ACAM-529 actually infects cells in the human body, but is unable to spread / propagate the infection (and thus is unable to cause disease). In the case of a whole HSV-2 vaccine, the virus has its own pathogen-associated molecular patterns that provide the “danger signals.” Thus, whole HSV-2 viral vaccines generate their own pro-inflammatory / immune-activating signals, and do not require additional adjuvants such as alum or MPL.

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B. The second ingredient in any vaccine is one or more foreign components, which are generally referred to as “antigens” or “immunogens.” This portion of vaccines is the most complex part of a HSV-2 vaccine and is, in my opinion, where the difference lies between (1) robust HSV-2 vaccines that offer complete protection against HSV-2 genital herpes versus (2) HSV-2 vaccines that are ineffective in animal models and in clinical trials (e.g., Herpevac).

The logic behind the terms “antigens” and “immunogens” is circular in nature, and so I will restrict my discussion of these terms to their functional significance.  Fortunately, the formal nomenclature of immunology is not necessary to explain the general concept that (1) the foreign components in a HSV-2 vaccine serve as activators of those B- and T-lymphocytes that happen to be HSV-2-specific; and (2) vaccine-induced expansion of these HSV-2-specific B- and T-lymphocytes radically increases the rate with which the immune system may recognize and suppress HSV-2 replication (before disease occurs). Hence, we say that an effective HSV-2 vaccine should confer upon the vaccine recipient the property of “immunity” to ever contracting the disease of HSV-2 genital herpes.

I think of the formal definition of “antigens” and “immunogens” as being analogous to road signs in Philadelphia. They make perfect sense once you have lived there for 2 years and know exactly where you are going, but are generally more confusing than helpful to newcomers. Specifically, the term “antigen” means any foreign component that stimulates antibody-generation when introduced into the human body, and the term “antibody” means a protein-based product of B-lymphocytes that binds tightly to the specific antigen that stimulated its generation. Like road signs in Philly, this should make perfect sense to anyone who has already studied immunology, and will probably sound like Greek to everyone else. For the purposes of this discussion, it will suffice to say that “antigens” are activators of the body’s B-lymphocytes and “immunogens” are activators of the body’s T-lymphocytes.

 

B-1. Background information. Before proceeding into a discussion of how a vaccine engages the human immune system, there are three pieces of background information that a reader must appreciate to fully appreciate how a specific vaccine may reduce the incidence of an infectious disease, such as genital herpes, that is caused by a specific microbial pathogen, such as HSV-2. These three pieces of information are:

(1) In the absence of vaccination, less than 20% of microbial infections of humans result in overt disease. Even in people who are immunologically naïve (i.e., who have not previously been exposed), most microbial infections don’t spread far enough or last long enough in the human body to produce the visible symptoms of infectious disease. This is certainly true for HSV-2, but is true for many other infectious agents as well.

(2) The difference between an asymptomatic HSV-2 infection versus a disease-causing HSV-2 infection largely reduces to the duration of HSV-2 replication and/or spread following a primary infection. Asymptomatic HSV-2 infections may be thought of as lasting for 2 – 4 days, and may lead to seeding of <100 peripheral nerve fibers with 1 to 50 copies of HSV-2 DNA per latently infected neuron. While this is still a “life-long, latent HSV-2 infection,” in quantitative terms this might represent less than 1% of the latent HSV-2 DNA load that exists in persons who experience disease-causing, primary HSV-2 infections that progress to recurrent genital herpes. By contrast, a disease-causing, primary HSV-2 infection might last 7 – 21 days, thus allowing far greater HSV-2 viral amplification and seeding of the peripheral nervous system. Such a latent HSV-2 infection might seed 1000s of peripheral nerve fibers (coming off the lower backbone) with 100s of copies of HSV-2 DNA per latently infected neuron. This >100-fold increase in the size of the reservoir of latent HSV-2 DNA is what places who people who experience symptomatic, primary HSV-2 infections at a >100-fold higher risk for a lifetime of recurrences of genital herpes relative to people whose first encounter with HSV-2 causes only an asymptomatic infection.

(3) The idea of a preventative HSV-2 vaccine is simple. In an unvaccinated population, 80% of people infected with wild-type HSV-2 will have no symptoms, whereas 5% of HSV-2 infected persons will fail to initially control the primary infection, and thus will be placed at risk for a lifetime of episodic recurrences of HSV-2 genital herpes. In contrast, in a population of individuals vaccinated with an effective HSV-2 vaccine (vaccinated before onset of sexual activity), essentially 100% of people would experience only asymptomatic infections, which last for 2 – 4 days, if exposed to wild-type HSV-2 later in life; this would be insufficient to produce the symptoms of genital herpes. Importantly, the load of latent, wild-type HSV-2 DNA in such persons would be too low to support recurrent genital herpes. Thus, an effective HSV-2 vaccine would prevent both primary and recurrent genital herpes caused by HSV-2, and would likewise prevent the downstream consequences of neonatal herpes and enhanced risk of HIV infection.

 

B-2. Why do less than 20% of infections produce visible disease in an unvaccinated population?

Our immune systems are critical to our survival, and constantly beat back microbes from the skin, mouth, and intestines. If this seems like an abstract idea, compare the human body in life and death. In life, you will look nearly the same four weeks from now. What would your body look like 4 weeks from now if your heart stopped beating? The human immune system is the difference. As long as your blood is flowing (and is loaded with antibodies and T-cells), then the microbial flora of your skin and intestines is kept in check. When a person passes away, and the blood quits flowing, the immune system stops functioning and our bodies are consumed by microbes in a matter of days to weeks.

Likewise, our immune systems gives us an innate ability to repel invading microbes such as HSV-2 (i.e., even in an unvaccinated population). By the numbers, this “innate immune response” is adequate 80% of the time to keep a primary HSV-2 infection from spreading in an uncontrolled fashion and causing symptomatic disease. However, 15% of the time, primary HSV-2 infections get far enough ahead of the innate immune system to produce some symptoms of herpetic disease. About 5% of the time, primary HSV-2 infections get completely beyond the control of our innate immune defenses, and these infections may last for 2 or 3 weeks and may set the infected person up for a lifetime of recurrences of HSV-2 genital herpes disease.

Once you appreciate that our “innate immune systems” (a few key cells are shown in the picture at the top of the post) reduce the burden of HSV-2 genital herpes in the world by about 10- to 20-fold, then the question becomes, “Why doesn’t our innate immune system do the job 100% of the time?” Our immune systems are a double-edged sword; they keep microbial invaders out of our bodies, but if our immune systems are overactive, then they may cause diseases in their own right such as Crohn’s disease, Grave’s disease, lupus, rheumatoid arthritis, and a myriad of other conditions where the immune system’s killing potential is turned against components of self (i.e., against tissues and cells of our own bodies).

Given that the human immune system is like a huge army with a tremendous killing potential, it is essential that our bodies tightly regulate this killing potential and selectively focus the ATTACK on only those things that enter our body that are foreign, such as microbial invaders like HSV-2. The way in which the human immune system achieves this delicate balance is that it has created a subset of cells, lymphocytes, whose sole function lies in enhancing the rate of RECOGNITION of that which is foreign. This is the primary function of our body’s B- and T-lymphocytes. Much of the potential of the immune system to ATTACK that which is foreign is embedded in the “innate immune system,” however the weakness of the innate immune system is that it is clunky, or inefficient, in its capacity to RECOGNIZE that which is foreign. Thus, left to its own devices, our innate immune system is slow to recognize and attack a primary HSV-2 infection; 80% of the time this innate immune response is adequate to control a primary HSV-2 infection, but 20% of the time HSV-2 wins the battle and spreads enough to cause some symptoms of herpetic disease.

 

B-3. Why does the human immune response to HSV-2 get more efficient over time?

In an unvaccinated population, people who contract a primary HSV-2 infection develop an improved capacity to RECOGNIZE and control the HSV-2 infection over time. This change (increase) in the efficiency of immune recognition of HSV-2 is due to an ~100- to 1,000-fold expansion in the numbers of initially rare HSV-2-specific B- and T-lymphocytes in the bloodstream and lymphoid organs.

When the body’s numbers of “HSV-2-specific lymphocytes” are small (1 per million lymphocytes), then these cells are too rare in number to initially contribute to the fight, and so your “innate immune response” does the best it can in the relative absence of  HSV-2-specific B- and T-lymphocytes.  As a person’s lymphocytes see (RECOGNIZE) the antigens and immunogens of the HSV-2 virus, two changes occur: 1. the RECOGNITION event is a growth stimulus that activates the sleeping lymphocyte out of its coma, and drives it to start proliferating giving rise to greater numbers of HSV-2-specific B- and T-cells; and 2. the RECOGNITION event activates a subset of HSV-2-specific B- and T-cells to become effector (fighter) cells that actively engage the enemy. B-cells contribute to the fight by by differentiating into antibody-secreting cells; antibodies are simply a soluble form of the “B-cell receptor” by which the B-cell RECOGNIZES foreign HSV-2 antigen. T-cells contribute to the fight in a variety of ways, but one of the more graphic is that some HSV-2-specific T-cells become killer cells (cytotoxic T-lymphocytes) that can deliver a death blow to any virus-infected cell that presents the RECOGNIZED component of HSV-2 to the T-cell, which it “sees” via its “T-cell receptor” .

Pulling back from these mechanistic details, the broad concept is that regardless of whether or not we have been infected with HSV-2, we all possess at least a small number of B- and T-lymphocytes that are specific for HSV-2, and in an uninfected person I would put that number at ~1 HSV-2-specific lymphocyte per million lymphocytes; the other 99.9999% of lymphocytes are specific for the myriad of other microbes that might enter your body.  Lymphocytes give your body’s immune system the capacity to adapt to (learn how to deal with) any infection you may experience; this is why lymphocytes are described as the body’s “adaptive immune system.”   The weakness of the adaptive immune system is that the handful of HSV-2-specific lymphocytes in an uninfected person are (1) metabolically asleep and need to be jarred out of their slumber to be useful, and (2) these rare HSV-2-specific lymphocytes are just too few in number to initially contribute to the fight during a primary HSV-2 infection.  As illustrated in the picture at the top of this post, the critical cells of our adaptive immune systems are specifically B-cells, CD4+ T-cells (helper T-lymphocytes), and CD8+ T-cells (cytotoxic T-lymphocytes).

When a person is infected with HSV-2, the antigens and immunogens in the HSV-2 virus engage the body’s B- and T-lymphocytes, and thus (1) wake them from hibernation and (2) drive the clonal expansion of rare HSV-2 specific T- and B-cells until they are no longer that rare (e.g., from ~1 per million to ~1 per thousand). That level of clonal expansion of HSV-2-specific lymphocytes requires at least 4-weeks because it takes 24 hours to one activated lymphocyte to give rise to 2 daughter cells, and another 24 hours for 2 cells to become 4 cells, etc, etc). At the end of that expansion of HSV-2-specific B- and T-lymphocytes, the body is 100- to 1,000-fold more efficient in its capacity to RECOGNIZE a HSV-2 infection, and so a person with pre-existing “acquired immunity” to HSV-2 is highly resistant to exogenous infection with a 2nd, outside strain of HSV-2.  The bottom line here is that more HSV-2-specific lymphocytes = far more rapid RECOGNITION of HSV-2 infection, and more rapid ATTACK of HSV-2 infected cells by all components of the innate and adaptive immune systems (which work together in our bodies).  When it comes to fighting the enemy, lymphocytes may be thought of as the directors / managers and the components of the “innate immune system” may be thought of as the worker bees that provide the brute force to ATTACK and get the job done.

 

B-4. What do the foreign components in a HSV-2 vaccine do for the human body?

As discussed above, the foreign components in a HSV-2 vaccine consist of 1. “HSV-2 antigens” (activators of the body’s HSV-2-specific B-lymphocytes) and 2. “HSV-2 immunogens” (activators of the body’s HSV-2-specific T-lymphocytes).

The purpose of a HSV-2 vaccine is simple. If the “innate immune response” is inefficient in its capacity to RECOGNIZE a HSV-2 infection (and thus the ATTACK is slow to develop), then the purpose of a HSV-2 vaccine is to bridge the gap. In an unvaccinated population, 20% of HSV-2 infected people will develop some symptoms of herpetic disease because the infection is essentially a race between (1) HSV-2 replication and spread versus (2) clonal expansion of initially rare HSV-2-specific B- and T-cells (which can stop HSV-2 replication and spread). An effective HSV-2 vaccine bridges this gap by artificially introducing a wide variety of HSV-2 antigens (B-cell activators) and HSV-2 immunogens (T-cell activators) into the human body, such that we may artificially increase our “adaptive immune response” to HSV-2 before it is actually required to combat the wild-type HSV-2 virus. In other words, by delivering all of the components of the HSV-2 virus (less the disease-causing potential) into a vaccine recipient, we may thus prime, boost, and increase the vaccine recipients’ number of circulating HSV-2-specific lymphocytes from very rare (1 per million) to very frequent (1 per thousand), such that their “adaptive immune response to HSV-2” can be pre-established prior to an actual  exposure to the disease-causing virus. In addition to the (1) vaccine-induced clonal expansion of HSV-2-specific B- and T-lymphocytes, another critical activity of HSV-2 vaccines is that they (2) establish large numbers of HSV-2-specific antibodies in the circulating blood and lymph, which may directly and immediately contribute to the ATTACK should a vaccine recipient contract an HSV-2 infection.

For these reasons, an effective HSV-2 vaccine delivered in a preventative manner to adolescent children (prior to the onset of sexual activity) would reduce these individuals’ risk of contracting HSV-2 genital herpes disease by several thousand-fold relative to their risk if left unvaccinated.  If an effective HSV-2 vaccine were delivered into a population of human beings, it would remain possible that these people could be infected with wild-type HSV-2 (an invisible, molecular event), but the incidence of such HSV-2 infections progressing beyond an “asymptomatic infection” to the symptoms of genital herpes would effectively drop to 0%.   This possibility of effective vaccine-induced control of HSV-2 genital herpes in the human population stands in stark contrast to the current situation in which 500 million to 1 billion people serve as carriers of latent HSV-2 infections, ~20 million people are newly infected with HSV-2 each year, and 5% of HSV-2 infected persons [~20 to 40 million people] suffer through a lifetime of recurrent genital herpes disease.

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IF HSV-2 VACCINES ARE SIMPLE, THEN WHY DO THEY KEEP FAILING IN HUMAN TRIALS?

The explanation offered above for how HSV-2 vaccines is relatively simple.  If HSV-2 vaccines are so simple, then why do our efforts to advance a HSV-2 vaccine keep missing the mark?  I think there is a simple and plausible answer to this question, and I offer it below.

A narrative emerged amongst immunologists and vaccinologists over 30 years ago that suggested that any infectious agent, such as HSV-2, could be thought of as a collection of “antigens” and “immunogens.”  While the concept of antigens and immunogens certainly explains how lymphocytes “see” (RECOGNIZE) an infectious agent, it is in my opinion that it is nothing short of a “leap of faith” to cross your fingers and believe that a single antigen (a single protein) may be extracted from HSV-2, and will be sufficient to serve as an effective HSV-2 vaccine.  In my book, this idea is up there with the tooth fairy and Santa Claus.

One of the many problems with this theory is that HSV-2 encodes at least 75 proteins, and thus the “antigen” in the failed Herpevac vaccine (a truncated form of gD) represents only 0.8% of the proteins that HSV-2 may encode. While gD may be an important antigen, I find it incredibly naïve to suggest that the other 99.2% of HSV-2’s proteins have absolutely nothing to contribute to an effective HSV-2 vaccine. Aside from my internal belief system, the available evidence supports this interpretation. Thus, a like contributor to the failure of past HSV-2 vaccines is that they present too few of HSV-2’s foreign antigens / immunogens to the B- and T-lymphocytes. Hence, HSV-2 vaccines such as Herpevac may only drive the activation and clonal expansion of ~3% of the body’s total repertoire of HSV-2-specific B- and T-lymphocytes that are available to contribute to vaccine-induced protection against HSV-2.