SARS: Biomarkers

The SARS associated coronavirus (SARS-CoV) is the cause of Severe Acute Respiratory Syndrome (SARS) and antibody produced against SARS-CoV is an important biomarker of the disease. Antibodies are produced by the immune system in response to infection. The body’s means for controlling viral infections is to eliminate both the virus and the host cell containing the virus, which is accomplished using a combination of innate, cellular and humoral immune responses. Humoral immunity involves the production of antibodies directed against the structural antigens on the surface of the virus and cell mediated immunity (T cells) promotes antibody production and kills infected cells. SARS is a new disease to humans so SARS-CoV antibodies will not be found in populations that have not been exposed to the virus.

After SARS-CoV was recognized as the causal agent of SARS in March 2003, the complete genomic sequence of SARS-CoV was quickly determined. The genomic sequence of SARS-CoV includes the nucleic acid sequences of the antigens presented by the virus for which antibodies are directed against. The genome of the SARS coronavirus is 29,727 nucleotides in length and is similar in organization to that of other coronaviruses. There are 11 open reading frames corresponding to the predicted polymerase protein (polymerase 1a, 1b), spike protein (S), small membrane protein (E), membrane protein (M) and nucleocapsid protein (N). Antibodies produced against SARS-CoV will recognize these proteins.

http://www-micro.msb.le.ac.uk/3035/Coronaviruses.html

RAPID DIAGNOSTIC TESTS
As a biomarker of disease, the detection of SARS-CoV antibodies using a diagnostic test such as the enzyme-linked immunosorbent assay (ELISA or EIA) is one method to test for SARS.

The current laboratory tests to detect SARS-CoV are:
1. Serum Antibody Tests

EIA (Enzyme immunoassay)
IFA (Indirect fluorescent-antibody)

2. Reverse transcription-polymerase chain reaction (RT-PCR)

Detects SARS-CoV RNA in clinical specimens (serum, stool, nasal secretions)
Viral isolation for SARS-CoV
Very difficult and time consuming

The RT-PCR and viral isolation detection methods involve the detection of the virus itself while the serum antibody tests rely on detection of biomarkers of the disease. Due to the recent recognition of SARS and consequent recent development of rapid diagnostic tests to detect SARS-CoV, there is still a high degree of uncertainty regarding the sensitivity and specificity of these tests. Currently, the EIA is the primary test being used by public health laboratories. However, a positive test is interpreted as previous infection with SARS-CoV since development of antibody requires a certain amount of time and some people do not test positive until more than 28 days after onset of illness.

SARS VACCINE?
There are multiple countries working to develop a vaccine to protect against SARS. Vaccines induce the production of antibody so if a vaccine for SARS is developed, and a person has been vaccinated, the presence of antibody will not necessarily indicate infection.

Knowledge of the genomic sequence is essential for the development of a vaccine against SARS. Since a vaccine must induce protective immunity in order to be effective, the most immunogenic portions of the virus need to be identified and incorporated into the vaccine. This means the vaccine should include the proteins from the virus that will induce the production of protective antibodies. The spike protein (S) and membrane protein (M) may be immunogenic antigens but sequencing has shown they have a high mutation rate. Mutating viruses like SARS-CoV complicate the production of an effective vaccine because antibody produced against antigens that are rapidly mutating will be ineffective and therefore not provide protective immunity.

INDUSTRY AND THE SEARCH FOR BIOMARKERS

Ciphergen Biosystems, Inc – Their protein chip technology is being used by multiple groups to identify protein biomarkers associated with SARS
http://www.biospace.com/news_story.cfm?StoryID=12878320&full=1
Imgenex – Developed antibodies against the spike and nucleocapsid proteins of SARS coronavirus
http://www.imgenex.com/antibody_list.php?id=20

www.cdc.gov/ncidod/sars/testresultsc.htm
http://english.peopledaily.com.cn/200305/05/eng20030505_116255.shtml
www.who.int/csr/sars/labmethods/en
Goldsby, Richard A. et al. Kuby Immunology. New York: W.H. Freeman and Company, 2000.
Murray, Patrick R. et al. Medical Microbiology. Mosby, 1998.

SEVERE ACUTE RESPIRATORY SYNDROME (SARS)