Conjugate Vaccines- Taming Meningococcus C

There are 6 serotypes of Neisseria meningitidis- A, B, C, X, Y and W-135, based on the capsular polysaccharide that imparts the organism its main defense against the host immune system- resistance against phagocytosis.

W-135 and X serogroups are more common in Africa. Group C accounted for 40% of meningitis cases in children in the UK pre-1999, when the conjugate vaccine against this organism was introduced. More about the vaccine in a moment.

The organism is one of the few that is vulnerable to direct lytic attack by the complement membrane attack system, comprising factors Vb-IX. The antibodies that set off the classical pathway for complement activation are unusual in this case, in that they are not produced by B cells that have been primed by helper T cells. Rather, the B-cells are involved through a thymus independent process and are capable of directly responding to the invading meningococci without help from the T cells. Thus, menigococcal antigens are an example of thymus-independent or TI antigens.

There are 2 types of TI antigens- TI-1 and TI-2. A prototype of TI-1 antigens are the lipopolysaccharides released from gram negative organisms, which are capable of activating both mature and immature B cells. In high doses, TI-1 antigens can activate all B cells, irrespective of their antigen specificity. They are thus called B cell mitogens. This represents an useful line of defense for the body against these usually virulent organisms before T-cell mediated adaptive immunity has had time to kick in.

The capsular polysaccharide present in N.menigitidis is an example of TI-2 antigen. Such antigens typically have repetitive epitopes and inactivate immature B cells. They can however activate mature B cells by cross linking the B cell receptors through their repetitive epitopes, but if the cross linking is extensive, even mature B cells can become anergic, as happens with very high levels of antigen.

It is thought that children under the age of two years have mainly immature B cells, which would explain why they have a poor response to vaccines containing the meningococcal polysaccharide. Thus, young children do not have the capacity to respond to TI-2 antigens, unlike adults. Unfortunately, they are also the most vulnerable to this potentially deadly infection.

The problem was solved through a stroke of brilliance by observing that by conjugating TI-2 antigens to an immunogenic protein such as tetanus toxoid, you could convert TI antigens into thymus dependent (TD) antigens. Such "conjugate" antigens are dealt with by T cells, which recognise the peptide fragments presented on MHC Class II molecules by B cells, and stimulate the B cells to produce antibodies against the peptide-polysaccharide complex. Such a strategy works in children.

The United Kingdom was the first country to introduce the conjugate Meningococcus C vaccine (Men C) in November 1999. Within a few years, the proportion of the C serogroup contributing to childhood meningeal infections fell from 40% to 10%. Other countries adopted the vaccine throughout Europe with gratifying results.

Unfortunately, no such vaccine is available against Meningococcus B, which now accounts for 80% of childhood meningitis in the UK. The biggest roadblock is the fact that the polysaccharide antigen in this serogroup resembles an antigen present in neurons in the human brain, thus raising the possibility that a vaccine might cause the body to produce autoantibodies against the brain. Various strategies are being explored to overcome this diificulty.

Conjugate vaccines have also been successfully used against another erstwhile common cause of childhood meningitis- Haemophilus influenzae, group B.

Curiously, the B-cells that are most active against TI-2 antigens are not your usual B-cells. They are two evolutionally primitive cells of B cell lineage called B-1 cells (so called because they were discovered ahead of the much more common "conventional" B cells or B-2 cells) and marginal zone B cells, so named because of their location in the spleen on the periphery of T cell areas. As stated before, these B cells do not depend on the helper T cells for activation, unlike conventional B cells. It is thought however, that they do receive co-stimulatory signals from dendritic cells in the form of BAFF (B-cell activating factor- also called BLyS or B-cell stimulator), which acts on a B cell receptor called TACI. Defects in the latter can lead to a condition called Common Variable Immunodeficiency, which has been discussed elsewhere on this blog.

The importance of TI-2 response is illustrated by sufferers of another X-linked condition called Wiskott Aldrich syndrome, who are incapable or mounting a response to TI-2 antigens, and are thus susceptible to infections by bacteria with polysaccharide capsules such as Meningococcus, Pneumococcus and H.influenzae.

The Hygiene Hypothesis

As somebody who grew up in India, it was instructive to see how common allergic disorders are in the west. Peanut allergy, ragweed pollen allergy, dust mite allergy... resulting in conditions such as anaphylactic reactions, hay fever and asthma are all too common. These disorders are relatively uncommon in countries like India.

An interesting hypothesis has been put forward to explain this phenomenon, called the hygiene hypothesis. It goes like this.

There are four types of CD4 or Helper T cells- Th1, Th2, Th17 and Treg. The direction that an inflammatory process, say due to an infection, takes is largely determined by whether the helper cells differentiate into Th1 or Th2 cells. Treg cells are the controller cells- they tend to suppress the other subtypes.

Th1 differentiation occurs in response to IL-12 and interferon-gamma, while Th2 lineage develops mainly in response to IL-4.

In allergy, the T cell that predominates is of the Th2 subtype. Several cytokines produced by these cells contribute to particular cell types or changes vital to allergy. Thus IL-5 leads to accumulation and survival of eosinophils, IL-4 and IL-13 cause isotype switching from IgM to IgE, rather than to IgG or IgA, while IL-3 and IL-9 augment mucus secretion. IL-4 also suppresses the development of the Th1 phenotype.

It is thought that in developing countries, the numerous childhood infections that kids are subjected to, steers the development of helper T cells towards the Th1 subtype. It is thought that cytokines that promote the Th1 phenotype, such as IL-12, also suppress the development of Th2 cells, and thus protect against allergic diatheses. This is the hygiene hypothesis.

There is a major spanner in the works with this hypothesis though. Children in developing countries also carry a much higher load of various parasites, which have been clearly associated with predominance of the Th2 subtype of T helper cells. Why do then such childen not have a higher prevalence of allergic disorders, which is also subserved by Th2 cells? In fact, parasitic infestation is associated with a lower prevalence of allergies. How does one explain this?

To account for this inconsistency, another hypothesis called the counter-regulatory hypothesis has been put forward. This states that any infections, whether associated with the Th1 phenoptype, such as common bacterial chest infections in childhood, or with the Th2 phenotype, such as is found with parasitic infestations, protect against the future development of allergy. It is thought that the body responds to such infections in the long run by increasing the 4th subtype of helper T-cells, called the T regulatory or Treg cells (which carry CD4 and CD25 surface molecules and have been mentioned elsewhere on this blog). Treg cells act by damping down both Th1 and Th2 phenotypes, mainly by secreting cytokines IL-10 and TGF-beta, and it is the suppression of the Th2 phenotype that protects against allergy.

The All Important X-Chromosome

Although the MHC genes reside on chromosome 6, a host of serious immunodeficiency disorders have been linked with mutations or deletions of genes on the X-chromosome. The X-chromosome is probably one of the most important from an immunological point of view. Naturally, all such disorders express themselves phenotypically in boys. As there is no normal allele to balance out these defects in males, the resulting immunodeficiency is severe, and manifests early in life.

Bruton described a X-linked syndrome with complete absence of immunoglobulins called X-linked agammaglobulinaemia (XLA). The disease is also named after him. The condition is because of a defect in a cellular tyrosine kinase, called btk, that halts the development of B-cells. Female carriers can be discerned due to the phenomenon of X-inactivation, described elsewhere on this blog. Thus, all circulating B cells will have been selected out by virtue of having inactivated the defective X-chromosome. OTOH, cells that do not depend on the defective gene for their function, such as T-cells, will have random inactivation of either X-chromosome, with a 1:1 ratio of such cells in circulation.

The earliest antibody isotype to be made by B-cells is IgM. Later, as the B-cell matures, en route to its ultimate role as the plasma cell, there is switching of the isotype to production of other antibodies, namely IgG, IgA or IgE. Along with this process of isotype switching, random mutations are introduced in the variable region of the immunoglobulin molecule, a process that is called somatic hypermutation. This process increases the affinity with which the B-cell receptor and its secreted immunoglobulin bind their putative antigen, a phenomenon called affinity maturation. The latter occurs only when the B-cell is exposed to its specific antigen in peripheral lymphoid tissues. This whole process cannot occur without the aid of helper T-cells, which are also constantly traversing peripheral lymphoid tissues, having entered through small blood vessels called high endothelial venules. The fact that an antigen specific B cell meets an antigen specific T cell is something of a small miracle- the odds of this happening has been calculated as being of the order of 10^-10. But meet they do, in the Tcell/Bcell border zones of lymph nodes and the spleen. T cells have on their surface a molecule called CD40 ligand (also called CD154). This ligates CD40 on B cells, providing a co-stimulatory signal that allows isotype switching and somatic hypermutaion to occur. It follows therefore, that in the absence of CD40-CD40L interaction, isotype switching will not occur, and the B-cells will continue to express one antibody isotype- IgM. This is exactly what happens in X-linked hyper-IgM syndrome, where there is a defect in the CD40 ligand on T cells. The interaction is bidirectional, and also activates T cells to express other co-stimulatory molecules. Thus, there is also a T-cell defect in this condition.

Other causes of hyper IgM syndrome have been identified. Predictably, defects in CD40 will give rise to a similar condition. A condition called NEMO syndrome leads to the same phenotype, as does deficiency of an enzyme called activation induced cytidine deaminase or AID, which is essential for the process of both isotype switching and somatic hypermutation. This latter only affects B-cells and thus causes a milder immunodeficiency than the first three aetiologies.

A further, much more severe immunodeficiency disorder has been characterised as "Severe Combined Immunodeficiency" or SCID. SCID has either X linked or autosomal recessive forms of inhertance. The X-linked form, which accounts for around 60% of cases, is due to deficiency of the gamma chain of the receptor of Interleukin-2 (IL-2). IL-2 is the most important survival and growth factor for T cells and is induced by co-stimulation of T cells through CD28 by B7 present on APCs, a process that's been described elsewhere on this blog. Upon activation, 3 transcription fators- nFkappaB, AP-1 and NFAT, are induced in the T cell, which leads to a hundred-fold surge in the production of IL-2. IL-2 is an autocrine cytokine- it acts on the T cell itself, through the IL-2 receptor. This receptor is trimeric when fully functional, and has alpha, beta and gamma chains. The beta and gamma chains exist as a dimer in the naive T cell. When the T cell is activated by its antigen and through co-stimulation, the beta-gamma heterodimer associates with the alpha chain, thus making a fully functional receptor, with a very high affinity for IL-2. This cannot happen in the absence of the gamma chain, which is defective in most cases of X-SCID. Other causes of X-SCID have been described including the absence of Janus Kinase-3 (JAK-3) and a defect in the gene that is responsible for the IL-7 receptor. In addition, autosomal forms of this condition exist due to deficiency of RAG-1 and RAG-2 recombinase enzymes which catalyse the somatic rearrangement of DNA in T and B cells that determine receptor specificity, a rare syndrome called ARTEMIS and deficiency of an enzyme called adenosine deaminase.

In addition to all these X-linked immunodeficiency disorders, an autoimmune X-linked condition has also been described. This is called IPEX (Immune dysregulation, Polyendocrinopathy, Enteropathy, and X-linkage). This condition is caused by a mutation in the gene for FoxP3, an essential transcription factor for natural Treg(CD4 CD25)cells, and results in severe allergic inflammation, autoimmune polyendocrinopathy, diarrhoea, haemolytic anaemia and thrombocytopaenia.

Since all the above conditions are recessive, female carriers of defective genes are phenotypically normal and do not manifest immunodeficiency.

The Mpemba Effect

Have you heard of the Mpemba effect? No, didn't think you would have.

Erasto Mpemba was a Tanzanian schoolboy in 1963 when he informed his science teacher that he could make ice-cream faster by putting hot milk in the freezer than when he used cold milk. He was laughed at for his troubles. The findings were confirmed by school inspector Denis Osborne, and the two published an article on this phenomenon in a peer reviewed journal in 1969.

Apparently, the effect is due to super-cooling of water. Cold water doesn't strictly freeze at 0 degree Celsius- it often takes much lower temparatures before it finally turns into ice. OTOH, hot water is not subject to supercooling. It needs to get only to 0 degrees to freeze. Thus, the rapidly cooling hot milk freezes faster than the cold milk, which lingers around the freezing point of water for a while.

The Mpemba effect is not always reproducible. Apparently, it only works under certain conditions.

Why Don't Women Mount A Immune Attack On The Foetus?

This is one of the oldest puzzles that have confounded Immunologists. The father, and thus the foetus, often contain major histocompatibility antigens that are distinct from those carried by the pregnant woman. Yet, the foetus is almost never immunologically attacked by the maternal immune system. There are several reasons for this.

The placenta is of foetal origin. The outermost layer of the placenta, called the trophoblast, is poor in both MHC Class I & Class II antigens, thus reducing the risk of attack by maternal T cells. However, not expressing MHC Class I antigens does open up the trophoblast to attacks from Natural Killer (NK cells). It does express a weakly antigenic MHC Class I molecule called HLA-G. This interacts directly with the two main inhibitory receptors on NK cells- KIR-1&2, and thus keeps the NK cells at bay.

Secondly, the placental tissue produces an enzyme called IDO- indoleamine 2,3 dioxygenase. This degrades the tryptophan contained in T cell proteins, and makes these T cells less likely to become activated.

Thirdly, it is thought the specific T cells that the mother does have against major and minor paternal antigens become temporarily anergic to these antigens. This tolerance disappears after the baby is delivered.

Fourthly, regulatory T cells such as CD4 CD25 T-reg cells may play a part by suppressing Th1 responses in particular. Thus, cytokines such as IL-4, IL-10 & TGF-beta predominate in the placenta, which steer the T-cell response away from the more stridently inflammatory Th1 phenotype to Th2 and T-reg phenotypes.

The foetus is not the only "tissue" that's exempt from immune attacks. There are three other sites in the body- namely the brain, the anterior chamber of the eye, and the testes, which are normally sequestered from the immune system. They are thus "immunologically previleged". The brain and the anterior chamber of the eye lack lymphatics, and the brain is also separated physically by the blood brain barrier. This sequestration does occasionally break down, resulting in autoimmune conditions such as multiple sclerosis, but in general, this phenomenon of immune previlege explains why HLA-matching is not required for transplants in these regions such as corneal grafts, and also why such grafts are quite long-lived.

No such tolerance exists outwith pregnancy. When organs such as kidney, etc are transplanted into the (MHC matched) female of the species from the male, the female's T-lymphocytes recognise the foreign proteins translated from genes on the Y chromosome of the male, and attacks them, thus increasing risk of graft failure. This does not happen with female--> male transplants, as both have common proteins translated from the mRNA transcripts of the X-chromosome. This phenomenon is known as H-Y, and explains why a solid organ or bone marrow transplant from female to male is more likely to succeed than a male to female transplant, although modern immuno-suppressive regimes have narrowed the gap.

The Amazing Mr HIV

HIV infects the CD4 T cells. But how does it enter those cells? It has two envelope glycoproteins called gp 120 and gp 41. These are translated from a single unspliced mRNA, which itself is transcribed from the env gene that forms part of the 9-gene HIV genome. The virus relies on a host protease to split the two envelope proteins. However, it needs its own protease to split apart another monolithic protein (called polyprotein), that comprises the viral core protein and another that includes vital enzymes like reverse transciptase, integrase and the protease itself (mRNA transcribed from two genes called gag and pol)- this viral protease is a target for therapeutic protease inhibitors that form a cornerstone of HAART (highly active anti-retroviral therapy).

Gp 120 binds to the CD4 molecule itself, but also to a chemokine receptor on the surface of CD4 T cells, called CCR5. CCR5 is normally a ligand for chemokines called CCL3, CCL4 and CCL5 and serves a vital function in normal T-cell physiology. After gp 120 has anchored the virus to its target CD4 T-cell, the second envelope protein- gp 41- causes fusion of the viral coat with the cell membrane of the T-cell, thus allowing the virus to enter its victim.

As you can expect, CCR5 is vital for virus replication. This has been starkly brought out by a small number of subjects of Caucasian origin, who are completely resistant to HIV infection, despite repeated exposure. As it happens, these individuals are homozygous for a mutated CCR5 exon, which has a 30 base pair deletion resulting in a frameshift mutation and a truncated CCR5 peptide chain, termed delta 30. The defective protein blocks entry for HIV into the CD4 T cell. Currently, there is intense research into developing a pharmacological CCR5 binding molecule that would block entry of HIV into CD4 T-cells.

Curiously, there is another portal for entry for HIV into the CD4 T-cell. This is a receptor called CXCR4 (The 2 Cs stand for cysteine residues, joined to each other by another amino acid, designated X). It is thought that CCR5 acts as the entry point for the virus early in infection, while CXCR4 becomes important much later in the course of infection, around the time that the infected individual develops clinical AIDS. HIV virus that uses the CCR5 co-receptor for entry is also called R5 virus, while virus subgroups that use CXCR4 are called X4 viruses.

Innate & Adaptive Immunity

Mammals have two distinct, but interrelated immune tiers in place- the primitive innate immune system, which they share with other vertebrates, and the much more evolved adaptive immune system. When the body is invaded by foreign pathogens such as bacteria or viruses, it is the innate immunity that kicks in first- this is a generic system, geared to kill pathogens in general, and lacks specificity. Meanwhile, the much more specialised adaptive immune system recognises the specific antigen and somewhat later, mounts a deadly directed assault which, more often than not, wipes out the pathogens.

An example of the workings of both innate and adaptive immunity can be found with the human immune response to viruses. Viruses lack the capacity to make their own protein, such as the one that constitutes their coat. They can only do so by invading host cells, incorporating their own RNA or DNA into the host genome and getting the host to make the viral proteins.

When viruses first attack mammalian cells, they alter certain glyco-proteins present on the cell surface called MHC Class I molecules. These are present on all cells. Viruses downgrade or alter the production of MHC class I molecules with a rather clever motive-these molecules are essential for the next step of the host defence- the adaptive immunity. However, a member of the innate immune system, called Natural Killer cells, or NK cells, has been put in place for just such an eventuality. The NK cells recognise the cells that have altered or diminished MHC class I molecules on their surface and directly kill them by releasing enzymes called granzyme and perforin, which cause the virus infected cell to die in an orderly fashion, called apooptosis. This process is highly directed, so that neighbouring cells don't suffer. Neighbouring healthy cells are also protected by increasing their expression of MHC class I molecules due to the influence of a group of soluble agents called Class I interferons- comprising alpha and beta-interferons. Although many cells produce these two interferons, the principal source are a group of cells called plasmacytoid dendritic cells, which produce 1000 times more class I interferons than any other cells. These interferons upregulate the production of MHC class I proteins by normal cells, thus protecting them from attack by NK cells. They also produce a distinct type of interferon themselves, called interferon-gamma, which facilitates later responses by the adaptive immune system, specifically by activating a class of T-cells called Th1 cells.

For those viruses which manage to evade the NK cells and survive in host cells, backup is required. Enter the cytotoxic T lymphocytes. T cells, or thymus derived lymphocytes, are broadly of 2 types- CD4 and CD8, and form pillars of the adaptive immune system. CD8 lymphocytes, also called cytotoxic lymphocytes, recognise specific foreign antigens, mainly viral, presented on MHC class I molecules by infected cells. (thus they work in an exactly opposite way to NK cells, which recognise cells with low levels of MHC class I molecules). Cytotoxic, or CD8 lymphocytes, kill in exactly the same way as NK cells, by releasing the same destructive enzymes onto the surface of the virus infected cell in a directed way, but being part of the adaptive immune system, they are much more specific. Instead of generically killing any virus infected cell, they recognise a specific virus as it sits within a host cell. They are thus much more specific and powerful killers than NK cells. Mice lacking MHC class I molecules tend to be overwhelmed by viruses, as CD8 cells require MHC class I molecules to recognise their specific viral targets.

We'll talk more about the interplay between the innate and adaptive immune systems in later posts.