The Ideal Vaccine: Until Death Do Us Part
Authors: Hildegund CJ Ertl
Journal: Molecular Therapy (2011)

Immunity induced by a vaccine should be like a happy marriage and last for the remainder of the vaccine recipient's life. Unfortunately, “divorce” rates are high—especially for very safe vaccines, such as those based on protein, although immune responses induced by more reactogenic live vaccines are less fickle. A recent article by Kasturi et al.1Kasturi SP Skountzou I Albrecht RA Koutsonanos D Hua T Nakaya HI et al.Programming the magnitude and persistence of antibody responses with innate immunity.Nature. 2011; 470: 543-547Crossref PubMed Scopus (750) Google Scholar offers a potential solution to this problem: a nanoparticle vaccine that induces high and sustained antibody titers as well as CD4+ and CD8+ T-cell responses in mice and nonhuman primates. This vaccine thus achieves a full spectrum of immune responses, which is generally induced only by infectious replicating pathogens. Does this report herald the coming of a new era in vaccinology that will finally allow us to produce efficacious vaccines to complex pathogens that continue to plague the world (such as HIV-1, Mycobacterium tuberculosis, or Plasmodium falciparum), or does it instead represent yet another shotgun marriage with little staying power? Only time will tell, but in the meantime, the work presents a well-executed series of experiments that offer food for thought. Kasturi et al. describe a nanoparticle vaccine composed of a biodegradable synthetic poly(d,l-lactic-co-glycolic acid) polymer. Nanoparticle vaccines based on a number of different chemical compounds release the entrapped molecules over a long period of time, and previous studies have shown them to induce potent B- and T-cell response.2Thomas C Rawat A Hope-Weeks L Ahsan F Aerosolized PLA and PLGA nanoparticles enhance humoral, mucosal and cytokine responses to hepatitis B vaccine.Mol Pharm. 2011Crossref Scopus (167) Google Scholar, 3Arias MA Loxley A Eatmon C Van Roey G Fairhurst D Mitchnick M et al.Carnauba wax nanoparticles enhance strong systemic and mucosal cellular and humoral immune responses to HIV-gp140 antigen.Vaccine. 2011; 29: 1258-1269Crossref PubMed Scopus (29) Google Scholar, 4Nagpal K Singh SK Mishra DN Chitosan nanoparticles: a promising system in novel drug delivery.Chem Pharm Bull. 2010; 58: 1423-1430Crossref PubMed Scopus (464) Google Scholar The authors added a new twist by mixing antigen-containing nanoparticles with nanoparticles containing ligands for the Toll-like receptors (TLRs) 4 and 7. Maximal responses were obtained if antigen-containing nanoparticles were mixed with those containing both TLR ligands. Entrapping all three together in a single particle or including only one of the TLR ligands was less effective. The optimal formulation enhanced germinal cell formation and induction of memory B and T cells, including CD4+ and CD8+ T cells. The quality of the response was also improved in that the elicited antibodies showed increased affinity to their cognate antigen, whereas CD8+ T cells had enhanced polyfunctionality. In addition, responses were highly sustained and, as one would expect, protected against challenge with a lethal pathogen. A formulation encoding hemagglutinin of influenza virus protected mice against challenges with very high doses of homologous virus. A hemagglutinin-containing vaccine also induced solid antibody titers in nonhuman primates, but, unfortunately, this part of the study did not assess protection against challenge. Enhancement of antibody and CD4+ T-cell responses required direct interactions between TLR ligands and B and dendritic cells. One of the more surprising findings was that the TLR ligandadjuvanted vaccine did not increase the number of short-lived antibody-secreting cells, which proliferate very rapidly after exposure to their cognate antigen, do not require germinal center formation, and produce antibodies of only low to moderate affinity. Instead, the vaccine enhanced long-lived plasma and memory B-cell formation, resulting in sustained production of high-affinity antibodies. Similarly, vaccine-induced T cells differentiated mainly into central memory cells, which allowed for their expansion upon booster immunization. Several aspects of the findings are important. From a practical standpoint, subunit vaccines based on purified protein are very safe. They induce antibody and CD4+ T-cell responses and upon repeated injections readily boost the immune response. As a rule, although there are exceptions, protein vaccines fail to induce CD8+ T-cell responses and may be more suited to protect against genetically stable pathogens, so that neutralizing antibodies induced by one surface protein protect against all circulating pathogens. Protein vaccines in general are not very immunogenic and thus require the addition of an adjuvant; even then, immune responses are commonly not sustained. In this sense, more potent and longer lasting responses by B and T cells, including those by CD8+ T cells, can be induced by subunit vaccines based on viral vectors—especially those that are nonlytic, such as E1-deleted adenovirus vectors,5Barouch DH Novel adenovirus vector-based vaccines for HIV-1.Curr Opin HIV AIDS. 2011; 5: 386-390Crossref Scopus (99) Google Scholar which persist for some time and thus allow for continued stimulation of the immune system.6Tatsis N Fitzgerald JC Reyes-Sandoval A Harris-McCoy KC Hensley SE Zhou D et al.Adenoviral vectors persist in vivo and maintain activated CD8+ T cells: implications for their use as vaccines.Blood. 2007; 110: 1916-1923Crossref PubMed Scopus (172) Google Scholar However, viral vector vaccines also induce neutralizing antibodies against the viral surface proteins, which reduce their effectiveness upon subsequent administrations.5Barouch DH Novel adenovirus vector-based vaccines for HIV-1.Curr Opin HIV AIDS. 2011; 5: 386-390Crossref Scopus (99) Google Scholar The formulation developed by Kasturi et al.1Kasturi SP Skountzou I Albrecht RA Koutsonanos D Hua T Nakaya HI et al.Programming the magnitude and persistence of antibody responses with innate immunity.Nature. 2011; 470: 543-547Crossref PubMed Scopus (750) Google Scholar appears to combine the advantages of a protein vaccine with those of a viral vector vaccine. Presumably, greater resistance to proteolytic degradation through incorporation of protein into nanoparticles contributes to enhanced immunogenicity,7Moon JJ Suh H Bershteyn A Stephan MT Liu H Huang B et al.Interbilayer-crosslinked multilamellar vesicles as synthetic vaccines for potent humoral and cellular immune responses.Nat Mater. 2011; 10: 243-251Crossref PubMed Scopus (449) Google Scholar whereas the added TLR ligands activate innate immunity, which in turn promotes stimulation of adaptive immune responses.8Kawai T Akira S The role of pattern-recognition receptors in innate immunity: update on Toll-like receptors.Nat Immunol. 2010; 11: 373-384Crossref PubMed Scopus (6345) Google Scholar TLRs are pathogen-recognition receptors expressed on the cell surface or within endosomes. When TLRs interact with a specific molecular pattern that is unique to pathogens or damaged mammalian cells, they signal through TRIF or MyD88, resulting in activation of nuclear factor kappa B or members of the interferon regulatory factor family of transcription factors, which in turn induces a proinflammatory cytokine response. In addition to the 9 or 10 TLRs that have been identified in mice or humans, respectively, other molecules within the cytoplasm, such as NOD-like receptors or RNA helices, can recognize pathogen-associated molecular patterns and activate an inflammatory reaction.8Kawai T Akira S The role of pattern-recognition receptors in innate immunity: update on Toll-like receptors.Nat Immunol. 2010; 11: 373-384Crossref PubMed Scopus (6345) Google Scholar,9Kanneganti TD Central roles of NLRs and inflammasomes in viral infection.Nat Rev Immunol. 2010; 10: 688-698Crossref PubMed Scopus (335) Google Scholar Many of these pathogen-recognition receptors have been tested as vaccine adjuvants,10Higgins SC Mills KH TLR, NLR agonists, and other immune modulators as infectious disease vaccine adjuvants.Curr Infect Dis Rep. 2010; 12: 4-12Crossref PubMed Scopus (41) Google Scholar but only monophosphoryl lipid A—a TLR4 ligand—has been approved for use as an adjuvant in human vaccines to date. Kasturi et al.1Kasturi SP Skountzou I Albrecht RA Koutsonanos D Hua T Nakaya HI et al.Programming the magnitude and persistence of antibody responses with innate immunity.Nature. 2011; 470: 543-547Crossref PubMed Scopus (750) Google Scholar incorporated ligands for TLRs 4 and 7. TLR4 is expressed on the cell surface, where it forms complexes with CD14 and Ly96. Upon interaction with lipopolysaccharide or its derivative monophosphoryl lipid A, TLR4 forms a homodimer and signals through MyD88 and TRIF. TLR7 is located in the endosome, is activated by single-stranded RNA, and signals through MyD88 (Figure 1). The study's results raise several questions about the mechanism of action of the two TLR ligands (Figure 2). First, it is curious that nanoparticles carrying the antigen together with the TLR ligands were less effective than two separate sets of nanoparticles, one carrying the antigen and the other the TLR ligands. Intuitively one would expect that TLR modulation of the very same dendritic cells and B cells targeted by the antigen would be most efficient. The authors describe their finding but fall short in offering an explanation, which frankly eludes this commentator as well. Second, stimulation of both TLRs was required to induce maximal responses. TLRs 4 and 7 both signal through MyD88; TLR4 also activates the TRIF pathway. One would have thought that TLR4, with its broader downstream signaling, would have sufficed and that addition of TLR7 would be redundant. Again, an explanation is not provided, but one could speculate that either the location of the two TLRs—TLR4 on the cell surface and TLR7 within the endosome—or differential activation of downstream mediators might have allowed for a synergistic effect. Third, the vaccine did not induce a strong effector cell response but drove differentiation of T cells toward memory cells and B cells toward long-lived plasma or memory cells. Microarray analyses conducted on antigen-experienced B cells from mice immunized with the various vaccine formulations indicated that the TLR ligands, rather than the slow release of antigen from the nanoparticles, drove memory B-cell formation. Again, the underlying mechanism remains elusive, although one might speculate that a particular cocktail of cytokines or chemokines affected the differentiation fate of the reacting lymphocytes. Other questions must be addressed to confirm or refute the broad applicability of this vaccine platform. Challenge experiments were conducted in vaccinated mice with a homologous influenza virus strain. Neutralizing antibodies are the known correlate of protection against influenza virus, and a vaccine that induces high-affinity neutralizing antibodies would be expected to provide protection. Protection against other infectious agents, such as M. tuberculosis, P. falciparum, and, potentially, HIV-1, may require CD8+ T cells. There is some experimental evidence, although this remains debated, that effector rather than resting memory CD8+ T cells provide superior protection against a rapidly replicating and mutating infectious agent.11Hansen SG Vieville C Whizin N Coyne-Johnson L Siess DC Drummond DD et al.Effector memory T cell responses are associated with protection of rhesus monkeys from mucosal simian immunodeficiency virus challenge.Nat Med. 2009; 15: 293-299Crossref PubMed Scopus (565) Google Scholar In addition, although the data are truly spectacular, further studies comparing the TLR ligandadjuvanted vaccine with other efficacious subunit vaccines, such as viral vector vaccines or DNA vaccines delivered by electroporation, are needed to gain a better perspective on the relative potency of this novel vaccine platform. The last sentence of the article by Kasturi et al.1Kasturi SP Skountzou I Albrecht RA Koutsonanos D Hua T Nakaya HI et al.Programming the magnitude and persistence of antibody responses with innate immunity.Nature. 2011; 470: 543-547Crossref PubMed Scopus (750) Google Scholar—“this vaccine formulation may provide a universal platform for vaccine design against pandemics and emerging infections”—evokes memories of initial reactions to naked DNA12Donnelly JJ Friedman A Martinez D Montgomery DL Shiver JW Motzel SL et al.Preclinical efficacy of a prototype DNA vaccine: enhanced protection against antigenic drift in influenza virus.Nat Med. 1995; 1: 583-587Crossref PubMed Scopus (287) Google Scholar and edible vaccines,13Mor TS Gómez-Lim MA Palmer KE Perspective: edible vaccines—a concept coming of age.Trends Microbiol. 1998; 6: 449-453Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar which both raised unrealistic expectations and after decades of improvement have yet to be licensed for use in humans. Clinical studies will show whether this new nanoparticle vaccine indeed performs as well in humans as in experimental animals as well as whether this nanoparticle-based immunological courtship ends with a happy marriage.