Evolution of Adaptive Immunity

Two Trees (creative digital image)

I attended a "Weiser Endowed Lecture in Immunology" featuring Dr Max Cooper of the Emory University School of Medicine, speaking on "Evolution of adaptive immunity" at the University of Washington in 2012.  Dr Cooper's work is foundational in this subject area.

It is interesting that immunity systems follow a phylogenetic pathway much as the phylogeny of species themselves do. The evolutionary pathway of jawless vertebrates (of whom hagfish and lampreys are surviving members), possess differences in comparison to the common evolutionary pathway followed by other vertebrates.

Lampreys and hagfish do not possess a thymus , nor do they possess a spleen.    Lampreys and hagfish do not possess Major Histocompatibility Complex (MHC) 1 or 2, T-cell receptors or recombination activating genes (RAG 1 and 2) which play an active role in the recombination of T-cell and immunoglobulin  receptors. Lampreys and hagfish posses hematopoietic tissue, the former in the intestine and the latter in the portal vein.  With these changes, the lampreys and hagfish diverged off the common immune system evolutionary pathway prior to the development of certain MHC class immunity in other species.  This is an important benchmark due to the major role that the MHC class plays in vertebrates.  It is a vital aspect which makes the concept of a separate phylogenetic pathway earmarked for the immune system so interesting.  If lampreys do not possess a thymus, then what substitutes in its place?

Lampreys and hagfish utilize a system of leucine-rich repeats )(LRR's) to mediate immune system responses using Variable Lymphocyte Receptor (VLRs), (VLRA's, VLRB's and VLRC's).  The interesting aspect of the jawless vertebrate immune system function is comparing its action in structure and function to the immune systems we are familiar with.  VLRB's have structures similar to toll like receptors (proteins that play a key role in the innate immune system), while their functions are similar to those of antibodies,  and they possess humoral characteristics (using macromolecules in extracellular fluids in immunity systems). They act using discrete populations of lymphocytes (types of white blood cells) .

VLRBs act as an adaptive immune system and can differentiate foreign from self in recognizing lymphocytes.

Lampreys posses a diverse variety of leucine-rich repeats which code for germline and mature genes. They possess a specificity for carbohydrate and protein receptors on bacterial and mammalian cells. They bind antigens with high avidity and affinity, describing the binding capacity of multiple versus single interactions of antibodies with antigenic epitopes.

 In "Evolutionary implications of a third lymphocyte lineage in lamprey", the authors find a thymoid source for VLRA and VLRC assembly in the lamprey gill tips, while VLRB assembly occurs in hematopoietic typhosole and kidney tissues.  VLRC appeared more numerous than VLRA, and predominated in the skin.  They conclude that a similar body plan of two T-cell type lymphocytes in a thymoid type structure and one B-cell type lymphocyte in the blood and kidneys shows some similarity in basic structure to the immune system for jawed vertebrate, while achieving adaptive immunity through different means.

Monclonal VLRB antibodies  from immunized lamprey larvae can recognize plasma cells from myeloma patients.  VLR4, a monoclonal VLRB specific for BclA, the bacillus collagen-like protein, has shown to be specific for B. Anthracis spores.  Information on B. Anthracis can be obtained from the US Centers for Disease Control (CDC) CDC - Anthrax and the World Health Organization (WHO) WHO - Anthrax.

Research indicates a number of applications where VLR's can recognize certain immumogens  An immunogen is any antigen that is capable of inducing humoral and/or cell-mediated immune response rather than immunological tolerance.  These applications include diagnosis, research and bioterrorism investigations.

PANSPERMIA and EVOLUTION

Mars Spirit Lander and Bonneville Crater in Color

 Image Credit: NASA/JPL-Caltech/Univ. of Arizona 

Panspermia  is a concept which portrays how life might be distributed throughout the universe.  These means include a wide variety of astronomical or celestrial objects  including meteors, comets, asteroids, and factors such as the solar wind.  The theory of panspermia, however does not really address how life began in the first place.

As man has ventured out into space, with human space travel, or has used unmanned spaceflight, the issue of man (or machine) as a vector or agent of panspermia has become an issue.  This issue parallels the concept of jet travel and airports as a vector in spreading contagion. Migratory birds have been vectors for centuries, landing in Qinghai Lake, China, a saline and alkaline lake and migratory crossroads  or in the Izembek National Wildlife Refuge in Alaska, along the Bering Sea. . Such bird migratory pathways provide fertile ground for recombination and spread of various contagions.

The NASA photo of the Bonneville Crater and Mars Spirit Lander depicts an impact crater on Mars.  Such an impact would have created a large amount of energy upon impact.  Many theories of life involve the discussion of how reorganization and complexity arises as energy is input into a system.  The Miller and Urey experiment in 1953 attempted to address this issue through recreation of a primordial atmosphere laced with water, methane and lightning storms.
 
Henry Eyring, a chemist, discovered Actual Rate Theory or Transition State Theory, which discusses reaction rates of chemicals in the context of potential energy states, complexes activated by chemical reactions, entropy considerations and products formed by the chemical reaction. In considering the input of exogenous energy into a system, this theory would be important in examining the potential reactions that might take place, as the reaction may change the system through transformational change.

In considering the issue of panspermia, therefore, there are a couple of issues; the development of a system and then the spread of that system.  Issues of energy and mixing concern the development or the boot-strapping of a system through transformational change, while panspermia addresses the spread.  Where panspermia occurs through media that spread via impact, the two issues (bootstrapping and spread) tend to co-exist to some extent.

The concept of panspermia requires a vector that is resilient.  It must be able to adapt to environmental factors such as extremes of temperature, must be able to resist impact as well as radiation and other factors such as high levels of salinity.   Such organisms, extremophiles  fit a profile as a candidate for panspermia.

Santa Catalina Mountains seen from Saguaro National Park, Arizona

Recently, I came across an article in Applied and Environmental Microbiology (AEM) on Bacillus Endospores,  written by Patricia Fajardo-Cavazos and Wayne Nicholson, "Bacillus Endospores Isolated from Granite: Close Molecular Relationships to Globally Distributed Bacillus spp. from Endolithic and Extreme Environments".   This article discusses the issue of the diversity of Bacillus spp. populations, and specifically certain endolithic spore populations obtained from near surface granite from the Santa Catalina mountains near Tucson, Arizona.

An endospore is a tough non-reproductive surface that lies dormant for an extended period of time, and can be later revived after many years to a vegetative state.

Anthrax is a bacillus, and may exist in the dormant state as an endospore.  According to a World Health Organization document on studies of anthrax endospores (bacillus anthracis),  "Sporulation requires the presence of free oxygen. In the natural situation, this means the vegetative cycles occur within the low oxygen environment of the infected host and, within the host, the organism is exclusively in the vegetative form. Once outside the host, sporulation commences upon exposure to the air and the spore forms are essentially the exclusive phase in the environment."  Spores are thus spread by release of spores by the dying host into the environment where they are taken up by another animal.

 The focus of the AEM article is on the very issue of panspermia, the issue of transference of microbes through spaceflight, It concerns the measurement of characteristics of various endolithic spore populations obtained through a variety of near surface granites and basalts and even the ultra clean environments of spacecraft assembly facilities.

Blue fluorescence obtained through medium wavelength UV illumination, distinguished certain endospores obtained from near surface granites.  Endospores were grouped into species or strains based on their rRNA  gene sequences. Numbers and diversity of endospores from granite were greater than those obtained from basalts.  This may be due to the lower porosity of basalt versus granite. Limited subsets of Bacillus spp. appeared to occur in certain rock environments that favored them and not other strains. Certain bacillus strains (bacillus subtilis) were used as proxies for bacillus anthracis fluorescence properties in the study.

The issue of panspermia is a very important one, set in the context of space exploration and in consideration of how extraterrestrial impacts may have impacted (and may still be impacting) human life.  The AEM study provides interesting results as to the diversity and characteristics of Bacillus spp. from a variety of environments, including, importantly, granite, set in the context of fluorescence properties.   A bacillus such as anthrax, as studied by the World Health Organization (WHO) or the CDC may provide clues as to how bacillus and endospores fit into our evolutionary framework.