Vaccination

The World Health Organization (WHO) estimate that vaccination averts 2-3 million

deaths per year (in all age groups), and up to 1.5 million children die each year due to

diseases which could have been prevented by vaccination. They estimate that 29% of

deaths of children under five years old in 2013 were vaccine preventable.

Vaccines: a success story in modern medicine

“with the exception of safe water, no modality, not even antibiotics, has had a major effect on mortality reduction and population growth” Plotkin ‘A Short History of Vaccination’ in Vaccines

Vaccines are one of the most important successes in modern

medicine and immunology The greatest triumph in vaccination has been the global eradication of smallpox,

announced by the World Health Organization in 1979.

What vaccines do • Most vaccines generate antibodies that neutralize the

pathogen and its associated toxins, and stop infection • pre-existing antibodies must already be present at the time

of infection (so vaccination must be performed before the subject is exposed to the pathogen) – Example: tetanus, for which you need antibodies to neutralize

the fast damage caused by the powerful toxin • Indeed the toxin is so powerful that disease inducing doses may not

be sufficient to induce an immune response, and survivors of tetanus need to be immunized anyway…

• Immune responses to infectious agents generate several different antibodies, but only some of them are protective – Vaccination strategy should optimize the selection of protective

antibodies

Features of effective vaccines • Close to 100% safe (even a low level of toxicity is not

acceptable) • Very effective: most if not all of the immunized

people must be protected • Long-lasting: it is not practical to immunize entire

populations repeatedly, the vaccine must generate memory

• Cheap

• Concept of “herd immunity”: as long as most of the individuals are immunized and therefore protected, not necessarily all of the individuals must be immunized (even unvaccinated members of the community are well protected, because the chance to encounter the pathogen is decreased)

• Anyway, 80% or >80% levels of immunized population are required, so that when levels fall below sporadic epidemics can occur

Limitations in vaccination strategies

• Vaccination works extremely well for diseases that can be efficiently tackled by a strong Ab-mediated response

• Many diseases (AIDS, malaria, tubercolosis) need both Ab- and cell-mediated immunity

• Current vaccination approaches are relatively weak in eliciting a strong cell response

Evolution in the development of vaccines

• Initial approaches – Use of organisms with reduced pathogenicity started as a purely

empirical approach, and evolved to the generation of genetically attenuated pathogens

• Functional genomics studies of the pathogens • Identify genes important for disease • Generate strain with mutated genes: unable or with dramatically reduced

ability to induce disease • Use these strains to vaccinate

– Alternative approach: use killed organisms, or purified components of organisms

• Example of “inactivated” purified component: use of inactivated toxins for tetanus

– Modern vaccinology: based on better understanding of pathogenicity/immunogenicity, response to pathogens, regulation of the immune system

•Vaccines must also be perceived to be safe. Bordetella pertussis causes whooping cough, which in small infants results in significant hospitalization (32% of cases), pneumonia (10% of cases)and death (0.2% of cases)

•The whole cell vaccine against Bordetella pertussis was developed in the 1930's and childhood vaccination in the US reduced the annual rate of infection from 200/100,000 in the 1940's to less than 2/100,000.

•Whole cell vaccine, given with tetanus and diphtheria toxoids, was associated with inflammation at the injection site. In a few children, high temperature and persistent crying occurred; very rarely, seizures or a transient unresponsive state were seen.

•Anecdotal reports that irreversible brain damage might be a rare consequence of pertussis vaccination, coupled with two deaths in Japan, lead to a decline in vaccination rates in the late 1970's and a rise in whopping cough and death due to pertussis infection, especially in Japan and in Great Britain.

•As a result of those 2 deaths in Japan that were feared to have been due to the vaccine, the vaccine was temporarily suspended, the given only to older children

•A few years later there was a big outbreak (13000 cases) and 41 kids died.

•Careful studies did not confirm that pertussis vaccination was a primary cause of brain injury, but in response to public concerns an acellular vaccine was developed containing purified antigens that induce protective immunity

•This vaccine is as effective as the whole cell vaccine and does not induce the common side-effects of the original vaccine.

•Recent anecdotal reports of association between childhood vaccination (particularly with MMR) and autism have raised concerns in parents; worldwide studies have found no association between the incidence of vaccination and autism.

Il vaccino MMR è un vaccino trivalente contro morbillo, parotite e pertosse. Nel 1998 un medico inglese Andrew Wakefield pubblica uno studio su Lancet, proponendo una possibile associazione tra campagne vaccinali e casi di autismo. Propone che i tre vaccini vengano somministrati separatamente

Nel Febbraio 2010, Lancet ritira la pubblicazione. Wakefield colpevole di frode scientifica

Figure 14-21

Immunity

• Active • Preferred, it induces immunological

memory • Passive

Active Immunization • Natural Infection with microorganism

or artificial acquisition (vaccine)

• Both stimulate the proliferation of T and B cells, resulting in the formation of effector and memory cells

• The formation of memory cells is the basis for the relatively permanent effects of vaccinations

Durable protection

Immunological memory

• At the cellular level, immunological memory depends on epigenetic changes in T lymphocytes and B lymphocytes that allow a faster effector response to pathogens. Immunological memory in B cells may also reflect genetic alterations as immunoglobulin M (IgM) antibodies switch to IgA or IgG and somatic mutations enhance antibody affinity.

There are two main kinetic patterns of expression for genes that are expressed at higher levels in memory T cells than in naive T cells. First, there are genes that are highly expressed in resting memory T cells compared with resting naive T cells. These highly expressed genes in resting memory T cells include genes involved in migration, homeostasis and readiness for activation. Second, there are genes that are highly expressed only after the activation of memory T cells; these genes are termed poised genes. Such poised genes are tightly regulated when the T cell is in the resting state but are rapidly induced after T cell activation. It is apparent that the function of these poised genes is not desired in the resting state, and therefore they are minimally expressed. These two patterns of expression for genes that are highly expressed in memory T cells show that the expression of such genes is precisely controlled in a time- and space-dependent manner to fulfil the function of memory T cells.

Vaccination strategies

Vaccines are made from: • Live micro-organisms that have been

‘treated’ so that they are weakened (attenuated) and are unable to cause disease.

• Dead micro-organisms.

• Some part or product of the

micro-organism that can produce an immune response.

Vaccine production.

Live-attenuated viral vaccines

• Many currently used antiviral vaccines (developed decades ago) consists of attenuated or inactivated viruses – Inactivated: treated to make them unable to replicate – Attenuated: grown by culturing the virus in vitro using

nonhuman cells, they become less able to grow in human cells

• They induce immunity but not disease • Compared to inactivated vaccines, they are >>>potent

because they elicit efficient CD4 and CD8 T cell responses

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Caveats of attenuated vaccines

• virus reactivation and induction of the disease – extremely rare, especially with new approaches to

attenuation

• In immuno-compromised individuals, attenuated viruses can be virulent and cause disease

New approaches to development of attenuated vaccines

• Identification of virulence genes – Introduce several mutations/delete the gene

• Extremely rare the reactivation

• Generation of an avirulent (nonpathogenic) virus that can be used to vaccinate

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Attenuated vaccines against bacteria and parasites

• BCG: protects children against disseminated tubercolosis, but not effective against the adult pulmonary disease

• Originally obtained from a pathogenic isolate of Mycobacterium bovis – More recent derivatives exploit recombinant DNA

technologies ( example: BCG overexpressing a M.tubercolosis-antigen)

Genetically attenuated parasites

• Sporozoites with key mutated genes cannot infect productively liver cells, but can circulate in the bloodstream and elicit an immune response that (preclinical studies) is protective against infection by WT sporozoites

Route of vaccination • It is important to consider the point of entry of the infectious agent

– Even if immunity is systemic, it is possible to have differences in the efficacy of the immune response in different organs, and therefore one goals it to achieve maximum efficacy at the usual point of entry for the pathogen (i.e. lungs for influenza virus, etc…)

• Usually vaccination occurs by injection – Intranasal vaccines are more effective to protect from upper

respiratory tract infections by influenza viruses – Injected vaccines are more effective to protect from lower respiratory

tract complications of the disease – Injection is expensive, requires trained people, etc….

• Orally administered antigens may induce tolerance rather than protection – it is important to understand the rules of mucosal immunity, a task

that is ongoing

Conjugate vaccines • Effort to produce acellular vaccines using isolated constituents of a

pathogen • A single constituent rarely if ever can be obtained from a single

antigen – Need to activate different types of immune cells to initiate an effective

immune response • Conjugate vaccines: example vaccines against H. influenzae type B,

responsible for meningitis – Vaccines directed against capsular polysaccharides

• Children <2 yrs of age cannot develop good antibodies (because they need T cells to develop protective antibodies, while in older children and adults protective antibodies against capsule polysaccharides do not need T cells)

– Bacterial polysaccharides have been coupled to protein carriers, that provide peptide antigens recognized by T cells

• Now there is an effective T cell response

Peptide-based vaccines • Approach to identify antigenic peptides:

– Empirical – Reverse immunogenetics (see tomorrow for more details)

• Peptide-based vaccines have drawbacks: – Difficult to identify antigens that are presented equally well by all MHC

molecules – Tolerance my be induced – MHC class I processing pathway is not involved, so activation of CD8 T

cells is restricted – Are not sufficient to stimulate the immune system, requiring

adjuvants • Development of new peptide vaccines (example new HPV vaccines)

– Use of very long peptides (100aa long), that can generate when processed several peptide epitopes that can be presented by different MHC molecules

Adjuvants

• Peptide-based vaccines require adjuvants to mimic how infection activates innate immunity, that induces dendritic cells to become optimally stimulatory for T cells

• Alum (inorganic aluminum salts) are approved in US for use as adjuvants, In Europe also squalene (an oil) is used – Alum activates NLRP3 (innate immunity bacterial

sensor mechanisms), thus inducing the inflammatory reactions that in the end lead to adaptive immune response

DNA vaccination

• Intramuscular injection of DNA (plasmids) encoding viral immunogens in mice-> – Umoral and cellular response that results in

immunization of the mouse

• In preclinical models, vaccination by DNA induces weak protection in many cases – Potentiation by plasmids expressing cytokines, or

co-stimulatory molecules

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