This past summer I had the opportunity to participate in a Summer Undergraduate Research Fellowship (SURF) at the Mayo Clinic in Rochester, Minnesota. The lab that I worked in (the Chella David lab) was a part of the Immunology Department, and specialized in determining the effects of different human leukocyte antigen (HLA) class II polymorphisms on the incidence of rheumatoid arthritis (RA) in humanized transgenic mice. My research involved examining the effects of aging on the immune system between different strains of these humanized mice.
Before I go on, I should provide some basic background about the parts of the immune system that I worked on. The major histocompatibility complex (MHC) class II molecules are found on professional antigen presenting cells (APCs) [like macrophages, dendritic cells, neutrophils, etc.], and act to present extracellular antigens in order to activate naïve helper T cells [CD4 T cells]. The HLA genes specifically code for MHC molecules in humans. There are three alleles associated with the HLA class II molecule: DR, DQ, and DP. These alleles are linked, and often occur in haplotypes. Within each allele are many different subtypes, making for the highly polymorphic nature of the HLA molecule. The lab where I worked over the summer genetically engineered single-transgenic mice strains to contain the DR*0401 allele and the DR*0402 allele, and double transgenic mice strains that contain the DR*0401.DQ8 haplotype and the DR*0402.DQ8 haplotype. When mice are injected with type II collagen (CII) [to cause collagen-induced arthritis (CIA), which is very similar to RA], the *0401 allele produces susceptibility to the disease, while the *0402 generates protection. The DQ8 allele by itself is also known to increase disease risk, but when in haplotype with the DR*0402 allele the mouse is protected. This pattern is also seen in human cases of RA.
All of this background information to ask one simple question: Is aging in the immune system associated with HLA polymorphism? To test this, naïve (non-immunized) mice either older than nine months or younger than five months were sacrificed and splenocytes were collected, which were used to find immune cell profiles through FACS (fluorescence activated cell sorter) flow cytometry analysis and immune responsiveness to different pathogenic/mitogenic proteins through proliferation assays. The supernatant from the proliferation assays was also screened for cytokines using ELISAs (enzyme-linked immunosorbent assays).
We found that aging in DR*0401 mice may lead to a skewed immune response, with high interleukin-6 (IL-6) and IL-13 levels and low immune response to stimulation. In DR*0402 mice, aging does not affect T cell response, and IL-13 levels are decreased. In addition, we found that during aging, haplotype positive mice seem to be unable to adequately produce IL-6 and IL-13. Aging also may lead to high B and T cell populations in DR*0401.DQ8 mice. The overall results of this study allowed us to conclude that age-related changes in immune response are indeed associated with HLA-polymorphism.
Before I go on, I should provide some basic background about the parts of the immune system that I worked on. The major histocompatibility complex (MHC) class II molecules are found on professional antigen presenting cells (APCs) [like macrophages, dendritic cells, neutrophils, etc.], and act to present extracellular antigens in order to activate naïve helper T cells [CD4 T cells]. The HLA genes specifically code for MHC molecules in humans. There are three alleles associated with the HLA class II molecule: DR, DQ, and DP. These alleles are linked, and often occur in haplotypes. Within each allele are many different subtypes, making for the highly polymorphic nature of the HLA molecule. The lab where I worked over the summer genetically engineered single-transgenic mice strains to contain the DR*0401 allele and the DR*0402 allele, and double transgenic mice strains that contain the DR*0401.DQ8 haplotype and the DR*0402.DQ8 haplotype. When mice are injected with type II collagen (CII) [to cause collagen-induced arthritis (CIA), which is very similar to RA], the *0401 allele produces susceptibility to the disease, while the *0402 generates protection. The DQ8 allele by itself is also known to increase disease risk, but when in haplotype with the DR*0402 allele the mouse is protected. This pattern is also seen in human cases of RA.
All of this background information to ask one simple question: Is aging in the immune system associated with HLA polymorphism? To test this, naïve (non-immunized) mice either older than nine months or younger than five months were sacrificed and splenocytes were collected, which were used to find immune cell profiles through FACS (fluorescence activated cell sorter) flow cytometry analysis and immune responsiveness to different pathogenic/mitogenic proteins through proliferation assays. The supernatant from the proliferation assays was also screened for cytokines using ELISAs (enzyme-linked immunosorbent assays).
We found that aging in DR*0401 mice may lead to a skewed immune response, with high interleukin-6 (IL-6) and IL-13 levels and low immune response to stimulation. In DR*0402 mice, aging does not affect T cell response, and IL-13 levels are decreased. In addition, we found that during aging, haplotype positive mice seem to be unable to adequately produce IL-6 and IL-13. Aging also may lead to high B and T cell populations in DR*0401.DQ8 mice. The overall results of this study allowed us to conclude that age-related changes in immune response are indeed associated with HLA-polymorphism.
This
research has been really interesting and exciting to me, because the
experiments I did are the David lab’s first pilot studies on the effects of
HLA-polymorphism on the aging immune system. The vagueness of my results just
goes to show how many different variables are involved in the aging process,
but with further refinement of the results I obtained, along with a greater
variety of tests and mice, we may be able to discern what kind of connection
exists between aging and HLA-polymorphism.
Taneja,
V. & David, C.S. (2010). Role of HLA class II genes in
susceptibility/resistance to inflammatory arthritis: Studies with humanized
mice. Immunological Reviews, 233, 62-78.
Mangalam,
A.K., Taneja, V., & David, C.S. (2013). HLA class II molecules influence
susceptibility versus protection in inflammatory diseases by determining
cytokine profile. The Journal of
Immunology, 190, 513-518.
Rene,
ReplyDeleteThat is some awesome research that you did with the polymorphisms. Although, I feel like it's common knowledge that as we age, it's easier for us to get sick. However, I guess the goal of the study is to nail down why, which it seems that this study was pretty successful in locating a potential cause. That's incredible. Hopefully with more studies geared toward finding results like these we can reduce the extent to which our immune systems degrade. Afterall, the best treatment in medicine is prevention.
Obviously, I got to researching a little bit, since we are all scientists here, and I stumbled across an article on PubMed, but I couldn’t find a way to view the whole thing. However, from what I gathered in the abstract and in one of the review articles, the telomeres (ends of the chromosome) degrade as cells multiply and DNA polymerase is constantly replicating (Weyand et. al 2009). After each replication, a small piece of the chromosome is lost, leaving less and less genetic material, which leads to mutations, disease, and the like. I wonder if there are some connections between what is going on with the immune polymorphisms and the loss of genetic material?
REFERENCES:
Weyand CM, Fujii H, Shao L, Goronzy JJ. 2009. Rejuvinating the immune system in rheumatoid arthritis. Nature Reviews: Rheumatology [Internet]. 5(10): 583-8. Available from: http://search.ebscohost.com/
I apologize for not responding sooner! Yes, indeed it is common knowledge that immune system functioning declines as we age, but what is not well understood is the role of HLA polymorphism in this decline. I am also hopeful that with further replicates of my initial studies and expansion of what is studied in particular [as well as narrowing down ages and defining parameters more specifically], we may be able to discern the role that HLA polymorphism plays in aging. However, based on my preliminary data all we can conclude is that age-related changes in immune response are associated with HLA polymorphism.
DeleteI think you are absolutely right, telomere degradation is a widely cited effect of aging on all somatic cells. The mutations that follow may explain why T cells lose expression of the CD28 costimulatory molecule, or why class switching occurs less frequently in B cells upon aging. I'm not entirely sure how these effects can be linked to variation in HLA polymorphism, since it is promiscuous binding to HLA class II molecules on APCs during thymic T cell maturation that often causes autoimmunity to occur. However, your point only emphasizes the complexity of the aging process, and how so many factors are involved that it is difficult to fully sort them out.
This comment has been removed by the author.
ReplyDeleteHi Rene,
DeleteCongratulations at getting an undergrad research fellowship at Mayo Clinic.
I have a question for you. You claim that aging may lead to high B and T cell populations in a certain population of mice. It seems that as one ages, one would acquire more B cells because we are continually exposed to new antigens throughout our lives, and would thus acquire a larger population of B cells to protect ourselves. This makes sense considering B cells are the cells that present the specialized anti-body receptors, but why would there be more T cells? Also, What is it about the DR*0401.DQ8 mouse that makes this result significant. I'm not well versed in genetics, although I find this result intriguing and I'm curious as to what the larger scale significance might be. I think sometimes in these genetic-intensive studies, significance gets lost in the details and results are therefore generally under appreciated. Can you help make sense of this for me?
I apologize for not responding to you sooner! It is true that with aging and increased exposure to primary infections, memory B cells accumulate in the bone marrow in case of any secondary infection. However, another phenomenon observed in the aging immune system is called oligoclonal expansion of memory phenotype T cells, which is the proliferation of T cells containing the same T cell receptors. Many of these T cells also do not express the important coreceptor CD28 and thus display aberrant activation in addition to lacking the receptor diversity necessary for identifying foreign antigen.
DeleteThe studies I conducted over the summer originated from unpublished data in which it was observed that DR*0401 and DR*0402 mice aged differently. Since these mice had only been used as a model to study rheumatoid arthritis, we decided to see if HLA polymorphism had any effect on the ways in which the immune system ages. If there was no effect, then the patterns of immune decline would be similar between all strains of humanized mice. However, this was not the case when DR*0401, DR*0402, DR*0401.DQ8, and DR*0402.DQ8 were compared.
The DR*0401.DQ8 haplotype is highly prone to autoimmunity, and is often associated with rheumatoid arthritis, type 1 diabetes, celiac disease, and multiple sclerosis. In contrast, the DR*0402.DQ8 haplotype confers protection from autoimmunity. If we can understand the differences between individuals with the DR*0401.DQ8 haplotype and the DR*0402.DQ8 haplotype, perhaps we can then understand how to treat individuals with these alleles. Comprehending the aging process in these genotypes may also elucidate some of the complexities associated with the aging process in general.
I hope this answered your questions - these were the first studies my laboratory had conducted on aging, and since I only had ten weeks to collect data, upon replication of my findings the conclusions and significance may change entirely. In addition, I only was able to test a male and a female mouse for each treatment group, which introduces error in the small test group size as well as the differences seen in aging between males and females. This is why I was only able to concretely conclude that age-related changes in immune response are associated with HLA-polymorphism. Now that we know this, we can delve further into the relation between HLA class II molecules and aging.