Extremely Low Genomic Diversity of Rickettsia japonica Distributed in Japan

Rickettsiae are obligate intracellular bacteria that have small genomes as a result of reductive evolution. Many Rickettsia species of the spotted fever group (SFG) cause tick-borne diseases known as “spotted fevers”. The life cycle of SFG rickettsiae is closely associated with that of the tick, which is generally thought to act as a bacterial vector and reservoir that maintains the bacterium through transstadial and transovarial transmission. Each SFG member is thought to have adapted to a specific tick species, thus restricting the bacterial distribution to a relatively limited geographic region. These unique features of SFG rickettsiae allow investigation of how the genomes of such biologically and ecologically specialized bacteria evolve after genome reduction and the types of population structures that are generated. Here, we performed a nationwide, high-resolution phylogenetic analysis of Rickettsia japonica, an etiological agent of Japanese spotted fever that is distributed in Japan and Korea. The comparison of complete or nearly complete sequences obtained from 31 R. japonica strains isolated from various sources in Japan over the past 30 years demonstrated an extremely low level of genomic diversity. In particular, only 34 single nucleotide polymorphisms were identified among the 27 strains of the major lineage containing all clinical isolates and tick isolates from the three tick species. Our data provide novel insights into the biology and genome evolution of R. japonica, including the possibilities of recent clonal expansion and a long generation time in nature due to the long dormant phase associated with tick life cycles.Citation: Akter A, Ooka T, Gotoh Y, Yamamoto S, Fujita H, Terasoma F, Kida K, Taira M, Nakadouzono F, Gokuden M, Hirano M, Miyashiro M, Inari K, Shimazu Y, Tabara K, Toyoda A, Yoshimura D, Itoh T, Kitano T, Sato MP, Katsura K, Mondal SI, Ogura Y, Ando S, Hayashi T. Extremely Low Genomic Diversity of Rickettsia japonica Distributed in Japan. Genome Biol Evol. 2017 Jan 1;9(1):124-133. doi: 10.1093/gbe/evw304. PMID: 28057731; PMCID: PMC5381555

Severe Fever with Thrombocytopenia Syndrome Virus Antigen Detection Using Monoclonal Antibodies to the Nucleocapsid Protein

<div><p>Background</p><p>Severe fever with thrombocytopenia syndrome (SFTS) is a tick-borne infectious disease with a high case fatality rate, and is caused by the SFTS virus (SFTSV). SFTS is endemic to China, South Korea, and Japan. The viral RNA level in sera of patients with SFTS is known to be strongly associated with outcomes. Virological SFTS diagnosis with high sensitivity and specificity are required in disease endemic areas.</p><p>Methodology/Principal Findings</p><p>We generated novel monoclonal antibodies (MAbs) against the SFTSV nucleocapsid (N) protein and developed a sandwich antigen (Ag)-capture enzyme-linked immunosorbent assay (ELISA) for the detection of N protein of SFTSV using MAb and polyclonal antibody as capture and detection antibodies, respectively. The Ag-capture system was capable of detecting at least 350–1220 TCID₅₀/100 μl/well from the culture supernatants of various SFTSV strains. The efficacy of the Ag-capture ELISA in SFTS diagnosis was evaluated using serum samples collected from patients suspected of having SFTS in Japan. All 24 serum samples (100%) containing high copy numbers of viral RNA (>10⁵ copies/ml) showed a positive reaction in the Ag-capture ELISA, whereas 12 out of 15 serum samples (80%) containing low copy numbers of viral RNA (<10⁵ copies/ml) showed a negative reaction in the Ag-capture ELISA. Among these Ag-capture ELISA-negative 12 samples, 9 (75%) were positive for IgG antibodies against SFTSV.</p><p>Conclusions</p><p>The newly developed Ag-capture ELISA is useful for SFTS diagnosis in acute phase patients with high levels of viremia.</p></div

Results of the Ag-capture ELISA in the qRT-PCR-positive serum samples.

<p>Results of the Ag-capture ELISA in the qRT-PCR-positive serum samples.</p

The relationship of the results with the Ag-capture ELISA and qRT-PCR tests using SFTS-suspected patient serum samples.

<p>The relationship of the results with the Ag-capture ELISA and qRT-PCR tests using SFTS-suspected patient serum samples.</p

Reactivity of MAbs (9D3 and 2D11) to SFTSV N protein and other TBPVs.

<p>(A) The indirect immunofluorescence staining (IFA) of MAbs. Vero cells infected with SFTSV strain YG1, RVFV strain MP12, FORV, and PALV were stained with MAbs. Rabbit sera obtained from animals immunized with SFTSV or RVFV recombinant N protein, or infected with FORV or PALV, were used as positive controls in the IFA. (B) The immunohistochemical staining of SFTSV antigens with the developed MAbs. The lymph nodes collected from patient with SFTS and patients without SFTS were used for evaluation of utility of the MAbs in SFTS diagnosis with the IHC analysis.</p