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18 public questions tagged with this topic.

Vaccine

Which chemical is commonly used to inactivate toxins in toxoid vaccines?

Toxoid vaccines prevent disease caused by potent bacterial exotoxins rather than bacterial invasion itself. Diphtheria toxin 58 kDa AB toxin ADP-ribosylates elongation factor eEF2 halting protein synthesis causing myocardial necrosis, neuritis; tetanus toxin tetanospasmin 150 kDa metalloprotease cleaves VAMP synaptobrevin preventing inhibitory neurotransmitter release causing spastic paralysis. Native toxins lethal at nanogram per kg doses far too dangerous for direct use. Formaldehyde formalin approximately 37 percent formaldehyde aqueous solution discovered by Gaston Ramon 1923 to detoxify toxin preserving immunogenicity. Mechanism involves cross linking lysine residues via hydroxymethyl intermediates forming methylene bridges disrupting catalytic site while preserving conformational epitopes recognized by neutralizing antibodies. Incubation with 0.2-0.6 percent formalin at 37°C pH 6-7 for several weeks converts toxin to toxoid losing toxicity but retaining ability to induce antitoxin antibodies that bind toxin preventing attachment to receptors. Verification by lack of toxicity in guinea pig challenge assay and absence of residual free formaldehyde. Toxoid adsorbed to aluminum salts generates long lasting IgG antitoxin, cornerstone of diphtheria tetanus control since 1920s, eliminating former leading causes childhood mortality.

Ref: Rappuoli Nat Rev Microbiol Toxoid formalin; Glenny 1920s Toxoid; WHO Toxoid production formalin.

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Vaccine

Inactivated vaccines usually require:

Inactivated vaccines usually require multiple doses and periodic boosters to achieve and sustain protective immunity because non replicating antigen provides transient immune stimulation. Initial dose primes naive B cells in draining lymph nodes producing low affinity IgM and modest IgG, small memory pool, titer often below correlate of protection threshold. Second dose administered 4-8 weeks later triggers anamnestic response with larger germinal center reaction, extensive somatic hypermutation in dark zone mediated by activation induced cytidine deaminase, affinity maturation, isotype class switching to IgG1 and IgG3 high neutralizing activity, expansion of long lived plasma cells homing bone marrow. Third dose further raises affinity. However antibody wanes over months to years because antigen depot cleared quickly unlike replicating live vaccine persisting weeks. Therefore boosters at 12-18 months and school entry maintain immunity. Examples hepatitis B series three doses at 0,1,6 months achieving seroprotection 95 percent, IPV three doses plus booster, DTP five doses. Reliance on multiple doses impacts compliance, coverage, programmatic cost, requiring tracking immunization records, reminder systems. Adjuvants reduce number but still need repeat exposures; oral live vaccines require single dose.

Ref: Plotkin Why inactivated needs multiple doses; WHO Immunization schedule; Amanna Duration.

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Vaccine

Inactivated vaccines differ from live vaccines because they:

Inactivated vaccines fundamentally differ from live attenuated vaccines because they cannot replicate within host tissues. Inactivation achieved by chemical agents formaldehyde crosslinking proteins, beta-propiolactone alkylating nucleic acids, heat denaturation destroying polymerase enzymes, while preserving epitopes for antibody recognition. Because genome destroyed, no progeny virions produced, infection cycle aborted, inability to cause disease even if host immunocompromised. Immunologically absence of cytosolic replication reduces engagement of RIG-I, MDA5 cytosolic sensors sensing replicating RNA, resulting in weaker type I interferon and CD8 T cell response primarily relying on exogenous antigen uptake presented via MHC II to CD4 helpers producing mainly humoral immunity. Duration shorter necessitating adjuvants like aluminum hydroxide forming depot, enhancing uptake by dendritic cells, activating NLRP3 inflammasome IL-1 beta release. Inactivated vaccines include IPV, HAV, influenza split, rabies. They exhibit increased safety, no shedding, no reversion, but require higher doses often multiple injections to achieve protective titers. Distinction impacts storage, contraindications and schedule design with more boosters required to maintain herd immunity threshold.

Ref: Baxter J Clin Virol 2007 Inactivated cannot replicate; Plotkin Inactivated vs Live difference; CDC.

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Vaccine

Which vaccine is administered orally?

Oral polio vaccine OPV is classical example administered via oral route as drops rather than injection. Formulated by Albert Sabin in 1957 containing live attenuated poliovirus types 1,2,3 propagated in primary monkey kidney cells, stabilized with MgCl2, sucrose, buffered. Oral delivery mimics natural fecal oral transmission of wild poliovirus. Attenuated virus replicates in M cells of Peyer's patches, gut associated lymphoid tissue generating strong intestinal secretory IgA preventing colonization and transmission, plus systemic IgG and intestinal CD4 T cells. Advantages include easy administration by volunteers without training, low cost, non sterile technique, secondary spread to contacts via fecal shedding increasing community immunity, cold chain still required but thermostable formulations being developed. OPV contributed to near eradication reducing polio cases 99 percent since 1988. However rare reversion of attenuating mutations in 5'UTR and VP1 capsid leads to vaccine derived poliovirus VDPV causing paralysis and circulating cVDPV outbreaks, prompting switch to inactivated IPV in many countries. Other oral vaccines include rotavirus pentavalent, cholera WC-rBS, typhoid Ty21a illustrating mucosal immunity advantages.

Ref: WHO OPV oral vaccine; Sabin J Exp Med 1957; Plotkin Polio oral route.

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Vaccine

Live attenuated vaccines are contraindicated in:

Live attenuated vaccines contraindicated in immunocompromised individuals because control of limited replication depends on intact innate and adaptive immune compartments. Immunocompromise includes congenital severe combined immunodeficiency affecting T cells, chronic granulomatous disease phagocyte defect, HIV with CD4 count below 200 cells per microliter in adults or below 15 percent in children, chemotherapy causing neutropenia, high dose systemic corticosteroids more than 2 mg per kg prednisone for 2 weeks, biologic agents anti TNF monoclonal antibodies, organ transplant recipients receiving calcineurin inhibitors tacrolimus suppressing T cell activation. In such hosts even attenuated strains can cause uncontrolled replication resulting in disseminated BCGitis with hepatosplenic granulomas after BCG, progressive vaccinia with necrotic lesions after smallpox vaccine, vaccine associated paralytic polio due to persistent poliovirus excretion and reversion, severe varicella from Oka vaccine. Pregnancy also contraindication due to potential fetal infection although risk theoretical. Therefore guidelines WHO advise use of inactivated, toxoid, subunit, conjugate, mRNA vaccines in immunocompromised which cannot replicate. Screening via HIV testing, medication history before live vaccine essential to avoid iatrogenic infection.

Ref: CDC Contraindications immunocompromised live; Plotkin Immunocompromised host; Red Book AAP.

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Vaccine

A major advantage of live attenuated vaccines is:

Major advantage of live attenuated vaccines lies in induction of strong, durable, broad immunity closely mimicking natural infection without causing disease in immunocompetent hosts. Limited replication provides continuous antigen synthesis over days to weeks prolonging immune exposure far beyond bolus injection of killed antigen, sustaining germinal center reactions, affinity maturation and generation of long lived plasma cells producing high affinity IgG persisting decades and memory B cells. Activation of cytosolic innate sensors RIG-I, MDA5, STING by replicating nucleic acids induces robust type I interferon, IL-12, potent CD8 cytotoxic T cell differentiation producing IFN gamma and perforin granzyme killing infected cells, important for viral clearance. Mucosal live vaccines like OPV and rotavirus elicit secretory IgA at portal entry preventing colonization transmission. Single dose often sufficient, sometimes lifelong, reducing programmatic complexity, compliance issues and costs compared to multiple booster schedules required for inactivated vaccines. Examples measles vaccine protection documented more than 20 years, yellow fever 30 years. However safety concern in immunocompromised due to potential progressive infection requires screening contraindications, outweighing advantage in vulnerable populations.

Ref: Amanna Nature Medicine 2007 Live vaccine long immunity; Plotkin Advantage robust; Pulendran.

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Vaccine

Vaccines primarily work by:

Vaccines function primarily by inducing immunological memory mimicking natural infection without causing severe pathology. Upon antigen administration intramuscularly or mucosally, local dendritic cells and macrophages sense danger signals via pattern recognition receptors TLR, NLR, STING. Activated dendritic cells pick up antigen, mature upregulating CCR7, migrate to draining lymph node T cell zone presenting peptides on MHC class II to naive CD4 T cells and via cross presentation on MHC I to CD8 cells. T follicular helper cells provide IL-21 and CD40L signals to B cells in germinal centers driving proliferation, somatic hypermutation of immunoglobulin variable genes mediated by AID, affinity maturation, class switch recombination to IgG and IgA isotypes, generation of long lived plasma cells homing to bone marrow secreting high affinity antibodies years, and memory B cells expressing surface Ig capable of rapid differentiation upon rechallenge. Memory CD4 and CD8 T cells persist as central memory and tissue resident populations secreting IFN gamma, IL-2. Upon natural exposure, anamnestic response within 2-3 days neutralizes pathogen preventing disease. Adjuvants enhance this process.

Ref: Pollard Bijker Nat Rev Immunol 2021 Vaccine memory; Janeway Immunobiology 9th ed Ch 12.

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Animal Tissue Culture 3

MRC-5 cells are derived from:

Death phase also called decline phase is final segment of in vitro growth curve where net cell number decreases as death rate surpasses division rate. Triggered by exhaustion of essential nutrients glucose and L-glutamine, severe acidification due to lactic acid production, accumulation of toxic ammonium from glutaminolysis, oxidative stress from reactive oxygen species generated by mitochondrial dysfunction, and activation of intrinsic apoptosis pathway involving Bax translocation, cytochrome c release from mitochondria, apoptosome formation with Apaf-1, and executioner caspase-3 cleavage of PARP. Senescent cells also accumulate expressing beta-galactosidase and senescence-associated secretory phenotype factors IL-6, IL-8, MMPs. Morphologically cells round up, detach, display membrane blebbing, chromatin condensation, and release cellular contents increasing debris. Debris promotes further damage via released proteases and DNases. In laboratory practice, allowing cultures to proceed into death phase compromises experimental results, selects for resistant subpopulations, and causes loss of valuable lines, underscoring importance of regular feeding and subculture before plateau extends.

Ref: Freshney Ch.13 Death phase decline apoptosis; Elmore S Toxicol Pathol 2007 Apoptosis intrinsic pathway caspase activation.

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Animal Tissue Culture 3

BHK21 cells are commonly employed in:

Plateau phase, also termed stationary phase, occurs when culture reaches carrying capacity constrained by surface area for adherent cells or nutrient limits for suspension cells. Cell number stabilizes as division rate equals death rate, curve flattening. Molecular mechanisms include contact inhibition mediated by Hippo pathway: high cell density activates LATS1/2 kinases phosphorylating YAP leading to cytoplasmic retention and degradation preventing transcription of cyclin D1 and survivin. Cadherin engagement also induces p27Kip1. Simultaneously growth factor exhaustion, accumulation of inhibitory metabolites lactate and ammonia lowering pH from 7.4 toward 6.8, and oxygen gradients limit proliferation. Cells remain metabolically active, can differentiate, for example myoblasts fuse into myotubes when growth factors withdrawn. Proliferation markers Ki-67 decline while quiescence markers increase. Viability remains high if medium replenished. Recognizing plateau timing guides differentiation protocols and indicates need for passaging if continued exponential growth desired, preventing accidental entry into death phase where toxicity rises. Optimizing extracellular matrix coatings such as collagen improves attachment reducing lag duration significantly. This knowledge strengthens laboratory safety, protocol reproducibility, and regulatory compliance critical for translational research and clinical applications, ensuring reliable data and workforce protection.

Ref: Alberts MBoC Ch.17 Plateau contact inhibition Hippo YAP; Freshney Ch.13 Growth curve plateau stationary quiescence balance.

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