Robustness against serum neutralization of a poliovirus type 1 from a lethal epidemic of poliomyelitis in the Republic of Congo in 2010

Abstract

In 2010, a large outbreak of poliomyelitis with unusual 47% lethality occurred in Pointe Noire, Republic of Congo. Vaccine-mediated immunity against the outbreak virus was never investigated. A wild poliovirus 1 (WPV1) isolated from a fatal case (termed PV1-RC2010) showed a previously unknown combination of amino acid exchanges in critical antigenic site 2 (AgS2, VP1 capsid protein positions 221SAAL → 221PADL). These exchanges were also detected in an additional 11 WPV1 strains from fatal cases. PV1-RC2010 escaped neutralization by three different mAbs relevant for AgS2. Virus neutralization was tested in sera from fatal cases, who died before supplementary immunization (n = 24), Gabonese recipients of recent oral polio vaccination (n = 12), routinely vaccinated German medical students (n = 34), and German outpatients tested for antipoliovirus immunity (n = 17) on Vero, human rhabdomyosarcoma, and human epidermoid carcinoma 2 cells. Fatal poliomyelitis cases gave laboratory evidence of previous trivalent vaccination. Neutralizing antibody titers against PV1-RC2010 were significantly lower than those against the vaccine strain Sabin-1, two genetically distinct WPV1s isolated in 1965 and 2010 and two genetically distinct vaccine-derived PV strains. Of German vaccinees tested according to World Health Organization protocols, 15-29% were unprotected according to their neutralization titers (<1:8 serum dilution), even though all were protected against Sabin-1. Phylogenetic analysis of the WPV1 outbreak strains suggested a recent introduction of virus progenitors from Asia with formation of separate Angolan and Congolese lineages. Only the latter carried both critical AgS2 mutations. Antigenetically variant PVs may become relevant during the final phase of poliomyelitis eradication in populations with predominantly vaccine-derived immunity. Sustained vaccination coverage and clinical and environmental surveillance will be necessary.

Fig. 1.

Neutralizing antibody titers against Sabin-1, -2, and -3 in fatal poliomyelitis cases from the RC2010 outbreak. (A) Thirteen laboratory-confirmed fatal cases. (B) Eleven additional fatal cases with clinical diagnosis of bulbar poliomyelitis and epidemiological linkage. Differences in the number of investigated sera are due to exhaustion of samples.

Fig. 2.

Serum neutralization against Sabin-1 compared with PV1-RC2010 on different cell culture systems. Endpoint serum dilutions still showing neutralization of PV1-RC2010 (x axis) or Sabin-1 (y axis) on Vero, RD, or HEp-2 cells. Cohorts are color-coded. Values are plotted with 10% jitter to prevent superimposition of datum points. Datum points still superimposing were shifted manually until they were distinguishable.

Fig. 3.

Deviations between serum neutralization against PV1 strains compared with PV1-RC2010. (A) Relative reciprocal neutralization titers on Vero cells against PV1-RC2010 (“α-RC2010”) divided by those against Sabin-1 (“α-Sabin”). (B) Titers on Vero cells against PV1-RC2010 and against Mahoney. (C) Confirmation of partial results from A on RD cells. (D) Confirmation of results from A and C on HEp-2C cells. (E) Titers against PV1-RC2010 compared with those against WPV1 Bar65 and Tajik; (F) against PV1-RC2010 and against two VDPV type 1 strains; and (G) against the two WPV1 and VDPV strains, respectively. All on RD cells for 31 sera from the student cohort shown in A–D. Filled circles represent null deviations.

Fig. 4.

WPV1 complete VP1 gene phylogeny including the RC2010 viruses. (Scale bar: 5 y; indicates evolutionary timing). Numbers at tree nodes indicate posterior probabilities (only values above 0.8 are shown). Numbers connected with a line to nodes A to F indicate node heights and 95% confidence intervals. African viruses are in red and Asian in blue. Viruses from the RC2010 outbreak are given in bold with patient codes. An expanded version of this phylogeny is shown in Fig. S4.