Human immunodeficiency virus type 1 (HIV-1) subtype C viruses have been found to almost exclusively use the chemokine receptor CCR5 as a coreceptor for entry, even in patients with advanced AIDS. We have characterized subtype C virus isolates from 28 patients from Harare, Zimbabwe, 20 of whom were receiving antiretroviral treatment. Virus from 10 of the treated patients induced syncytium formation (SI virus) when cultured with MT2 cells. Only non-syncytium-inducing (NSI) virus was cultured from the peripheral blood mononuclear cells of the eight patients who had not received treatment. The majority of these subtype C SI viruses were capable of using both CCR5 and CXCR4 as coreceptors for viral entry, and the consensus V3 loop sequences from the SI viruses displayed a high net charge compared to those of NSI viruses. While those on treatment had reverse transcriptase (RT) and protease mutations, there was no clear association between RT and protease drug resistance mutations and coreceptor tropism. These results suggest that CXCR4-tropic viruses are present within the quasispecies of patients infected with subtype C virus and that antiretroviral treatment may create an environment for the emergence of CXCR4 tropism.

HIV-1 drug resistance mutations have been identified and characterized mostly in subtype B HIV-1 infection. The extent to which antiretroviral drugs select for drug resistance mutations in non-subtype B HIV-1 is not known. We obtained HIV-1 reverse transcriptase (RT) and protease sequences from 21 Zimbabwean patients failing antiretroviral drug therapy. We compared these sequences with 56 published RT and protease subtype C sequences from untreated patients, 990 RT and 1140 protease subtype B sequences from treated patients, and 340 RT and 907 protease subtype B sequences from untreated patients and identified four mutation categories of subtype C HIV-1. Seventeen of the 21 patients (81%) had known drug resistance mutations. Mutations at 15 RT and 11 protease positions were more common in subtype C isolates than in subtype B isolates. HIV-1 subtype C-infected individuals receiving antiretroviral therapy develop many of the known subtype B drug resistance mutations. Comparison of subtype C RT and protease sequences with a large database of subtype B sequences identified subtype C-specific polymorphisms and candidate drug resistance mutations.

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Interest in the long-term low temperature preservation of malaria parasites was initiated by Coggeshall's (1939) demonstration that the intra-erythrocytic stages could survive storage at −76°C for 70 days. Numerous studies have subsequently been published in which a variety of techniques have been used. In addition, several reviews have included sections on the cryopreservation of malaria, the most recent being by James (1980), Leef, Hollingdale, & Beaudoin (1981), Nguyen-Dinh, Chemangey-Masaba, Spencer, Campbell, Chin & Collins (1981) and a WHO memorandum (1981). Most of these reports and reviews have dealt with the problems of cryopreserving malaria parasites from a parasitological viewpoint. In an attempt to provide a cryobiologically orientated review of value both to cryobiologists and practicing parasitologists the published technical reports have been summarized in chronological order in Tables 1 and 2, and the important cryobiological parameters are discussed.

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