In the present study, we evaluated the intra-subtype neutralization sensitivity of HIV-1 B’ isolates to the neutralization antibodies in the plasmas derived from patients from whom the viruses were isolated. We demonstrated that these isolates possess high intra-subtype neutralization sensitivity to the heterologous plasmas, but very low intra-subtype neutralization sensitivity to the autologous plasmas, which is consistent with the results from our previous study on the HIV-1 CRF07_BC recombinant
. These findings suggest that most HIV-1 B’ viruses can induce neutralization response, but they become resistant to autologous neutralizing antibodies, most likely by the immune escape mechanism developed during disease progression. By analyzing the neutralizing activity of consecutive patient plasma against the sequentially obtained autologous virus, Arendrup et al.
 found that the development of neutralization to the primary HIV-1 isolates was accompanied by emergence of escape virus with corresponding reduced neutralization sensitivity to autologous plasma. This may explain the apparent failure of the immune system to control HIV infection. In this study, we also showed that HIV-1 B’ isolates exhibit low inter-subtype neutralization sensitivity to the antibodies in the plasmas derived from the HIV-1 CRF07_BC-infected patients (Table
2), which may caused from low cross-reactive neutralizing activity in different subtype or the different neutralization potency in same subtype.
According to previous reports, the antibody response matures and becomes cross-neutralizing against divergent HIV-1 in up to 10% of HIV-1-infected patients, as shown in neutralization assays
[40, 41]. Interestingly, we found that the antibodies in some HIV-1 B’-infected patients have potent cross-neutralizing activity. 5 of 12 subtype B’ plasma samples showed broad neutralization activity against more than 80% of Env-pseudotyped reference viruses that covered HIV-1 clade B, C and A. And specially, the antibodies in AH259P could potently neutralize infection of PBMCs by a series of primary HIV-1 isolates with different subtypes (Figure
5b), while the AH259V virus was shown to be sensitive to neutralization by two known bnmAbs in this study (Figure
5a). VRC01 failed to neutralize AH259V, AH691V and AH096V at a concentration of 10μg/ml (Table
1). It is reported that many HIV-1 strains in China were resistant to PG9, PG16 and VRC01, which known to possess exceedingly high potency and breadth against diverse viruses from outside China
. These findings suggest that although most HIV-1 B’ viruses have low inter-subtype neutralization sensitivity, some of these primary HIV-1 isolates are able to induce both intra- and inter-subtype cross-neutralizing antibody responses.
It is reported that X4 HIV-1 viruses are generally more sensitive than R5 viruses to neutralizing antibodies
. Most HIV-1 B’ primary HIV-1 isolates collected from the FPDs were R5 viruses, while a few of them were dual-tropic R5X4 viruses. In our study, no significant difference in intra- and inter-subtype neutralization sensitivity between the R5 and R5X4 viruses was observed, suggesting that these R5X4 viruses may predominantly use CCR5 co-receptor for infection.
Positive correlation was shown between neutralization sensitivity of the HIV-1 B’ isolates and viral load in patients from whom the viruses were isolated, but no significant correlation between viral neutralization sensitivity and CD4+ T cell count in these patients was observed. Notably, the neutralization sensitivity of HIV-1 B’ isolates is inversely correlated with neutralizing activity of plasmas from the HIV-1 B’-infected patients (Figure
3), and with the number of PNGS in the V1-V5 sub-regions of gp120 (Figure
5d). This finding agrees with the reports of others showing that the decreased neutralization sensitivity of HIV-1 isolates results from increased glycan numbers in the viral env used to build up a “glycan shield” to protect the virus from attack by neutralizing antibodies
[44, 45]. Interestingly, increase in the number of PNGS is generally located in the gp120 variable regions of the neutralization-resistant isolates. AH259V, for instance, has 10 PNGS in V1-V2 region, while other viruses (e.g., AH1597V) with less resistance have 5 PNGS in the same region.
Dhillon et al.
 characterized the neutralization antibody present in the plasma of three individuals infected with subtype A and B viruses and found that the highly conserved neutralizing epitope in gp41 and highly variable V3 loop in gp120 may not account for the broad neutralizing activity, while the CD4 binding site in gp120 may be responsible for eliciting the neutralization antibody. We will use similar approaches to further investigate how the HIV-1 subtypes predominant in China induce intra-subtype neutralizing antibody response and protect themselves from attack by the autologous neutralizing antibodies. These studies may provide a sound scientific platform for the rational design of vaccines that induce antibody responses against the HIV-1 isolates circulating in China.