Electrophoretic analyses of protein components of striated muscle myofibril purified from various vertebrate and invertebrate species revealed that proteins much larger than myosin heavy chain are present in significant amounts. To define possible roles of these heretofore unidentified proteins, we purified a combination of two uncommonly large proteins, designated as titin, from chicken breast myofibrils. Chemical and immunological studies indicated that titin is distinct from myosin, actin, and filamin. Specific titin antibody crossreacts with similar protein in both skeletal and cardiac myofibrils of many vertebrate and invertebrate species. Immunofluorescent staining of glycerinated chicken breast myofibrils indicated that titin is present in M lines, Z lines, the junctions of A and I bands, and prhaps throughout the entire A bands. Similar staining studies of myofibrils from other species suggest that titinlilce proteins may be organized in all myofibrils according to a common architectural plan. We conclude that titin is a structurally conserved myofibrillar component of vertebrate and invertebrate striated muscles. The composition and organization of vertebrate skeletal myofibril have been the subject of intense studies (for recent reviews, see refs. 1 and 2). Detailed investigations of the properties of purified myofibrillar proteins have provided the molecular basis for understanding muscle contraction (3) and a framework to explore nonmuscle contractility (4, 5).Because our studies on the structure and function of a different contractile protein (6-8) had led to the unexpected observation that filamin is abundant in smooth muscle and a wide range of nonmuscle cells but is essentially absent in striated muscle (6, 7), we decided to undertake a comparison of the total protein compositions of striated and smooth muscle. In this investigation we found that the high molecular weight (Mr > 200,000) components in these two types of muscle differed significantly. Besides the virtual absence of filamin in striated muscle, each type of striated muscle that we studied was found to contain a group of three extremely large polypeptides (Mr > 400,000) that are not present in smooth muscle. By using chicken breast muscle as a source of striated muscle, we have studied the chemical and immunological properties and the distribution in myofibril of a combination of two of these polypeptides. This paper gives a general survey of our results. A preliminary report was presented at the 18th American Society for Cell Biology meeting (9). and smooth muscle [chicken gizzard muscle (lane c)] were blended in ice cold buffer (0.5 g of muscle per ml of 0.6 M KC1/5 mM EDTA/ 100 mM Tris-HCl, pH 7.0). The homogenates were solubilized in NaDodSO4 and electrophoresed on 4% polyacrylamide gels as described (7). Sufficient sample was used in lanes a and b to show a trace protein band with the same mobility as filamin. Purified striated muscle myofibrils [chicken breast muscle (lane d) and rabbit back muscle (lane e)], prepared a...

Passively transferred neutralizing antibodies can block lentivirus infection, but their role in postexposure prophylaxis is poorly understood. In this nonhuman-primate study, the effects of short-term antibody therapy on 5-year disease progression, virus load, and host immunity were explored. We reported previously that postinfection passive treatment with polyclonal immune globulin with high neutralizing titers against SIVsmE660 (SIVIG) significantly improved the 67-week health of SIVsmE660-infected Macaca mulatta macaques. Four of six treated macaques maintained low or undetectable levels of virus in plasma, compared with one of ten controls, while two rapid progressors controlled viremia only as long as the SIVIG was present. SIVIG treatment delayed the de novo production of envelope (Env)-specific antibodies by 8 weeks (13). We show here that differences in disease progression were also significant at 5 years postinfection, excluding rapid progressors (P ‫؍‬ 0.05). Macaques that maintained <10 3 virus particles per ml of plasma and <30 infectious virus particles per 10 6 mononuclear cells from peripheral blood and lymph nodes had delayed disease onset. All macaques that survived beyond 18 months had measurable Gag-specific CD8 ؉ cytotoxic T cells, regardless of treatment. Humoral immunity in survivors beyond 20 weeks was strikingly different in the SIVIG and control groups. Despite a delay in Env-specific binding antibodies, de novo production of neutralizing antibodies was significantly accelerated in SIVIG-treated macaques. Titers of de novo neutralizing antibodies at week 12 were comparable to levels achieved in controls only by week 32 or later. Acceleration of de novo simian immunodeficiency virus immunity in the presence of passively transferred neutralizing antibodies is a novel finding with implications for postexposure prophylaxis and vaccines.

A reliable method for the quantitation of plasma viremia in nonhuman primates infected with simian immunodeficiency virus (SIV) and related viruses is described. This method is based on an established quantitative-competitive PCR format and includes a truncated control for internal assay calibration. Optimization of assay conditions has significantly improved amplification specificity, and interassay variability is comparable to that of commercially available assays for human immunodeficiency virus (HIV) quantitation. This procedure was used to monitor viral loads in a group of Macaca mulatta animals that were infected with SIVsmE660 for over 2 years. Highly diverse profiles of plasma viremia were observed among animals, and high viral loads were associated with more rapid disease progression. Spearman rank correlation analyses were done for survival versus three parameters of viral load: plasma viremia, p27 core antigen, and frequency of infected peripheral blood mononuclear cells. Plasma viremia had the strongest overall correlation and was significantly (P < 0.05 to P < 0.01) associated with survival at 10 of the 13 time points examined. Plasma viremia did not correlate with survival during the primary viremia phase; however, the strength of this correlation increased with time postinfection and, remarkably, viremia levels as early as week 6 postinfection were highly predictive (P < 0.01) of relative survival. These findings are consistent with the available clinical data concerning viral load correlates early in HIV infection, and they provide further support for the view that disease outcome in lentiviral infection may be largely determined by events that occur shortly after infection.

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Reports of significant reductions in plasma viral load by anti-HIV drugs have raised the possibility that antiviral therapy, if initiated sufficiently early, may result in sustained control of infection and prolonged clinical benefits. We evaluated the effects of intervention coincident with infection using an antiviral nucleoside, d4T, in Macaca nemestrina infected with a highly pathogenic isolate of HIV-2 (HIV-2[287]). Infection with this virus reproducibly results in high viremia and rapid CD4+ cell depletion, allowing a sensitive measurement of the treatment effect on viral load and clinical outcome. Compared to the control group, d4T-treated macaques showed significantly lower (2-3 log10) plasma- and cell-associated viral load. No CD4+ cell decline was observed in the treatment group while on therapy with d4T whereas CD4+ cells of control macaques declined from a preinfection mean of 32% of PBMCs to below 10%. Notably, when d4T treatment was withdrawn after 16 weeks, five of the six macaques continued to control viral load and have maintained normal CD4+ cell level for more than a year. These results demonstrate that early antiviral intervention, even of a limited duration, may constitute an important strategy against lentiviral-induced disease.