Hristo Taskov - Academia.edu (original) (raw)
Papers by Hristo Taskov
PubMed, 1976
The authors study the cellular activity of RES against the microbes intracellular and extracellul... more The authors study the cellular activity of RES against the microbes intracellular and extracellular parasites in rats with pronounced Progeria-like syndrome of Selye. RES of the experimentally aged animals copes, through with a certain delay, with the invading extracellular bacterium Diplococcus pneumoniae, but is utterly helpless against the intracellular microbe Br. abortus 19. These relations are completly inverted in the rats that have not been experimentally aged.
PubMed, 1976
The authors study the activity of RES in rats with Progeria-like syndrome of Selye, at the occasi... more The authors study the activity of RES in rats with Progeria-like syndrome of Selye, at the occasion of a repeated infection with bacteria--intracellular parasites (Brucella abortus 19) and bacteria--extracellular parasites (Diplococcus pneumoniae). They establish that immunization improves the activity of RES of the experimentally aged rats; still, its phagocytic and digestive functions remain by far feebler than those of the rats unsubjected to experimental ageing. By the aged animals the RES insufficiency is more pronounced towards the intracellular bacterium Brucella abortus 19, than towards the extracellular Diplococcus pneumoniae.
Biomedical Reviews, Dec 31, 1994
• The ABH blood group determinants are carbohydrates found in different human cells and tissues. ... more • The ABH blood group determinants are carbohydrates found in different human cells and tissues. The modulation in their expression in the course of ontogenesis, as well as the studies on their subcellular localization, impose a discussion on the role of ABH antigens as molecules participating in cell interactions, contact inhibition, adhesion and metastases. It is supposed that they are also involved in cellular differentiation during human embryogenesis. The described association between blood groups and disease and the de novo acquisition of blood group determinants support the idea that ABH antigens are related to pathological processes. The modulation in blood group antigen expression observed in several tumors demands a clarification of their diagnostic and prognostic value as tumor-associated markers.
Hybridoma, Aug 1, 1994
A novel murine monoclonal antibody (MAb) 1D8 was produced after immunization with the nylon wool-... more A novel murine monoclonal antibody (MAb) 1D8 was produced after immunization with the nylon wool-adherent fraction of human peripheral blood mononuclear cells (PBMNC). Its reactivity pattern was studied on a panel of hemopoetic normal cells, cell lines and malignancies. Mab 1D8 detects a lymphocyte-specific surface antigen expressed on a major subpopulation of mature B lymphocytes as well as on a minor T-cell subset. Activation-related changes in the expression of 1D8 molecule were observed on both B and T lymphocytes. As compared with the pattern of known activation-associated antigens, 1D8 seems to play a role in the early stages of lymphocyte activation. The antigen could not be immunoprecipitated by the conventional methods for glycoproteins.
Brain Sciences
The first epidemiological wave of the incidence of COVID-19 in Bulgaria was registered in June 20... more The first epidemiological wave of the incidence of COVID-19 in Bulgaria was registered in June 2020. After the wave peak, we conducted a study in persons diagnosed with COVID-19 (N = 52). They were assessed with the anxiety–depressive scale (ADS), including basic (BS), vegetative (VS), conversion (CS), obsessive–phobic (OPS), and depressive (DS) symptoms. ADS assessment of individuals diagnosed with SARS-CoV-2 indicated a correlation between OPS and IL-33 values. IL-10 levels were higher than reference ranges in all patients. Multiple linear regression analyses demonstrated that combination of CS and OPS explained 28% of IL-33 levels, while combination of symptoms from all ADS dimensions explained 24% of IL-33 levels. It was also found that 21% of IL-28A levels was explained from the combination by all ADS dimensions, whereas OPS was the predictor for lower concentrations. The obtained results revealed meaningful correlations between psycho neuro–immunological factors in pathogenesi...
Folia Medica
Minimal residual disease refers to a leukemia cell population that is resistant to chemotherapy o... more Minimal residual disease refers to a leukemia cell population that is resistant to chemotherapy or radiotherapy and leads to disease relapse. The assessment of MRD is crucial for making an accurate prognosis of the disease and for the choice of optimal treatment strategy. Here, we review the advantages and disadvantages of the available genetic and phenotypic methods and focus on the multiparametric flow cytometry as a promising method with greater sensitivity, speed, and standardization options. In addition, we discuss how the application of automated data analysis outweighs the use of complex combinations of windows and gates in classical analysis, thus eliminating subjective evaluation.
Zentralblatt für Pathologie, 1993
The parallel expression of ABH blood-group antigens and of the carcinoembryonic antigen was exami... more The parallel expression of ABH blood-group antigens and of the carcinoembryonic antigen was examined applying the biotin-streptavidin immunostaining system. Monoclonal antibodies to the ABH antigens newly produced by us and a polyclonal antibody to the carcinoembryonic antigen (Ortho) were used as primary antibodies. Human tumours derived from six different organs were studied. ABH antigens showed a heterogeneous expression. They were entirely missing in some neoplastic tissues and found in single or in clustered tumour cells in others. The staining for the carcinoembryonic antigen revealed stronger intensity and covered large malignant areas. The possible functions of ABH blood-group antigens as tumour-associated structures are discussed. A number of tumour-associated antigens have been identified which may serve to improve the early diagnostics of tumour processes in man. Recently, the blood-group antigens (BGA) from the ABO/H system have attracted the attention of many investigat...
American Journal of Reproductive Immunology, 1995
PROBLEM: To test the relative impact of epididymal versus ejaculated sperm in immunologic inferti... more PROBLEM: To test the relative impact of epididymal versus ejaculated sperm in immunologic infertility.METHOD: Human antibody binding to epididymal and ejaculated spermatozoa was compared by flow cytometry (FCM) since it allows quantitative analysis of viable sperm while ignoring nonsperm cells. To select sera for FCM, GAT, TAT, and ELISA were applied on 145 sera from fertile men, idiopathically infertile and varicocele patients.RESULTS: All GAT/TAT‐positive infertile patients, a representative group of varicocele patients and the fertile control, were assessed by FCM. Higher reactivity toward epididymal sperm revealed 18/22 sera while only four out of them bound to ejaculated sperm stronger than the control. All varicocele sera were positive against epididymal while negative against ejaculated spermatozoa.CONCLUSIONS: Epididymal sperm antigens may play a predominant role in some cases of immunologic infertility. Such patients might not be adequately diagnosed and respectively treate...
Biologicals, 1993
The present study assesses the influence of the immunophenotyping procedure, the type of monoclon... more The present study assesses the influence of the immunophenotyping procedure, the type of monoclonal antibody (MAb) and flow cytometer used, upon the reproducibility of the results of estimation of the normal values for CD3, CD4, and CD8 positive lymphocyte subpopulations. A hundred healthy adults were immunophenotyped by direct immunofluorescence with commercially available (OKT3-FITC, OKT4-PE and OKT8-PE), and locally prepared (HL3-FITC, HL4-PE and HL8-PE)MAb. The samples were analysed on FACStar flow cytometer. The maximal values of the analytical (CD3-0.96, CD4-1.05, CD8-1.02), the individual (CD3-2.33, CD4-2.88, CD8-2.86), and the population (CD3-8.7, CD4-8.8, CD8-7.5) variation prove that the precision of immunophenotyping procedure has no effect on the variation of the values of the CD3+, CD4+ and CD8+ subpopulations. The values of all three markers showed Gaussian type of distribution. Regression analysis proved that the values of these subpopulations determined by MAb from two sources were highly correlated (r > 0.9) but the paired t-test gave statistically significant difference in the values of CD8 (1.23%, P > 0.001). Mean, SD and 95% range of the sizes of the positive subsets determined by OKT3, OKT4 and OKT8 showed that the results were very close to the results from a multicenter study of adult Caucasians. The values we find fall within the cited critical range for these markers; so we can conclude that if good care is taken for the precision of the procedure, MAb source and type of flow cytometer do not influence the values of the assayed subsets.
European Journal of Immunology, Apr 1, 1979
In vivo treatment with antineoplastic compounds has been reported to lead to the expression of ne... more In vivo treatment with antineoplastic compounds has been reported to lead to the expression of new antigenic specificities which were not detected on parental cells, and which were transmissible as a genetic character. The current study is concerned with antibody-dependent cellular cytotoxic (ADCC) activity in serum of syngeneic mice challenged with LY/DTIC cells, a subline of LY murine lymphoma, antigenically altered by the drug DTIC. LY/DTIC target cells coated with LY/DTICimmune serum were specifically lysed by virgin lymphocytes. The genetic background of the effector cells, whether syngeneic, allogeneic or xenogeneic, did not produce significant differences in the percentage of target-cell lysis. ADCC activity was reduced when the immune serum was added directly to the incubation medium, without precoating. Although sera from individual animals exhibited different levels of ADCC activity, they nevertheless followed the general trend of the pooled sera. Peak activity of ADCC was obtained in the sera collected on Days 8 and 30 after LY/DTIC cell challenge. The ADCC activity elicited by LY/DTIC cells may contribute to the rejection of drug-altered tumour cells.
PROBLEMS of Infectious and Parasitic Diseases
Background: SARS-CoV-2, the virus responsible for COVID-19 pandemic, has posed huge global health... more Background: SARS-CoV-2, the virus responsible for COVID-19 pandemic, has posed huge global health challenges. Understanding the immune response to SARS-CoV-2 infection, and in particular – the role of B cells in the generation of immune memory is crucial for assessing the durability of protective immunity. Materials and Methods: In this longitudinal prospective study, individuals who had recovered from SARS-CoV-2 infection were included. Peripheral venous blood samples were collected at three time intervals post symptom onset (PSO): 1-3 mo, 4-8 mo, and 9-12 mo. The humoral immune response was evaluated by measuring anti-SARS-CoV-2 IgG, virus-neutralizing antibody activity, total S1-specific B-cells, and B cell subpopulations. Results: The levels of anti-SARS-CoV-2 specific IgG antibodies decreased from 390.3 to 204.5 BAU/ml in the first 6-8 months PSO but did not significantly decrease further until the 12 th mo (126.6 BAU/ml). Virus-neutralizing antibodies (activity decreased by 20...
Folia medica
Human blood group antigens are genetically determined structures of undefined nature found not on... more Human blood group antigens are genetically determined structures of undefined nature found not only in red blood cells but also in various tissues. Monoclonal antibodies against A and B human blood group antigens (B7H4 and D1E8) were obtained. Their tissue compatibility was tested in 11 different normal human organs taken from people with A1 and B blood group phenotypes. The antigen-antibody reaction was visualized using the indirect immunoperoxidase technique. The results of the study showed that the antibodies B7H4 and D1E8 react specifically with the tissue blood group antigens in healthy subjects. The findings obtained in most of the tested organs were in accord with those described by other researchers. Differences were found only in the stomach, urothelium, the lungs and the thyroid gland. A possible explanation of the results is the difference in the epitopes recognized by the monoclonal antibodies used by us and the blood group antibodies applied by other researchers. It is ...
Tackling the World's Fastest-Growing HIV Epidemic: More Efficient HIV Responses in Eastern Europe and Central Asia, 2020
Bulgarian Journal of Agricultural Science, 2000
Zeitschrift für Alternsforschung, 1976
The authors study the morphology of the cells in the peritoneal exsudate and the phosphatase acti... more The authors study the morphology of the cells in the peritoneal exsudate and the phosphatase activity of the peritoneal macrophages, obtained from rats with Selye's Progeria-like syndrome. Judging by their morphologic characteristics the macrophages of the experimentally aged rats, before and after contact with Brucella abortus 19 do not differ from those obtained from the unsubjected to experimental ageing rats. The acid phosphatase activity and the adenosinetriphosphatase activity of the macrophages prior to contact with Brucella abortus 19 are nearly identical for both groups of animals. Following contact the activity of the enzymes increases but this increase is slower by aged rats and reaches its maximum 1--2 days later.
Folia medica, 1994
The human ABH blood-group antigen expression of cytokeratin, S-100 protein, and the melanoma-asso... more The human ABH blood-group antigen expression of cytokeratin, S-100 protein, and the melanoma-associated antigen was studied in 12 cases of pigmented nevi. The biotin-streptavidin immunostaining system was applied. ABH antigens were found both in the endothelial cells and in the germinative epidermal layer. The flat overlaying epithelium, single melanocytes and melanoblasts were positive for cytokeratin. Some basilar cells and the giant multinucleate cells in the Spitz nevi stained for S-100 protein. The melanoma-associated antigen was visualized in the upper epidermal layers as well as in some melanocytes. The present article discusses the specificity of the antigens studied as well as some immunologic traits of potential malignant development of Spitz nevi.
PubMed, 1976
The authors study the cellular activity of RES against the microbes intracellular and extracellul... more The authors study the cellular activity of RES against the microbes intracellular and extracellular parasites in rats with pronounced Progeria-like syndrome of Selye. RES of the experimentally aged animals copes, through with a certain delay, with the invading extracellular bacterium Diplococcus pneumoniae, but is utterly helpless against the intracellular microbe Br. abortus 19. These relations are completly inverted in the rats that have not been experimentally aged.
PubMed, 1976
The authors study the activity of RES in rats with Progeria-like syndrome of Selye, at the occasi... more The authors study the activity of RES in rats with Progeria-like syndrome of Selye, at the occasion of a repeated infection with bacteria--intracellular parasites (Brucella abortus 19) and bacteria--extracellular parasites (Diplococcus pneumoniae). They establish that immunization improves the activity of RES of the experimentally aged rats; still, its phagocytic and digestive functions remain by far feebler than those of the rats unsubjected to experimental ageing. By the aged animals the RES insufficiency is more pronounced towards the intracellular bacterium Brucella abortus 19, than towards the extracellular Diplococcus pneumoniae.
Biomedical Reviews, Dec 31, 1994
• The ABH blood group determinants are carbohydrates found in different human cells and tissues. ... more • The ABH blood group determinants are carbohydrates found in different human cells and tissues. The modulation in their expression in the course of ontogenesis, as well as the studies on their subcellular localization, impose a discussion on the role of ABH antigens as molecules participating in cell interactions, contact inhibition, adhesion and metastases. It is supposed that they are also involved in cellular differentiation during human embryogenesis. The described association between blood groups and disease and the de novo acquisition of blood group determinants support the idea that ABH antigens are related to pathological processes. The modulation in blood group antigen expression observed in several tumors demands a clarification of their diagnostic and prognostic value as tumor-associated markers.
Hybridoma, Aug 1, 1994
A novel murine monoclonal antibody (MAb) 1D8 was produced after immunization with the nylon wool-... more A novel murine monoclonal antibody (MAb) 1D8 was produced after immunization with the nylon wool-adherent fraction of human peripheral blood mononuclear cells (PBMNC). Its reactivity pattern was studied on a panel of hemopoetic normal cells, cell lines and malignancies. Mab 1D8 detects a lymphocyte-specific surface antigen expressed on a major subpopulation of mature B lymphocytes as well as on a minor T-cell subset. Activation-related changes in the expression of 1D8 molecule were observed on both B and T lymphocytes. As compared with the pattern of known activation-associated antigens, 1D8 seems to play a role in the early stages of lymphocyte activation. The antigen could not be immunoprecipitated by the conventional methods for glycoproteins.
Brain Sciences
The first epidemiological wave of the incidence of COVID-19 in Bulgaria was registered in June 20... more The first epidemiological wave of the incidence of COVID-19 in Bulgaria was registered in June 2020. After the wave peak, we conducted a study in persons diagnosed with COVID-19 (N = 52). They were assessed with the anxiety–depressive scale (ADS), including basic (BS), vegetative (VS), conversion (CS), obsessive–phobic (OPS), and depressive (DS) symptoms. ADS assessment of individuals diagnosed with SARS-CoV-2 indicated a correlation between OPS and IL-33 values. IL-10 levels were higher than reference ranges in all patients. Multiple linear regression analyses demonstrated that combination of CS and OPS explained 28% of IL-33 levels, while combination of symptoms from all ADS dimensions explained 24% of IL-33 levels. It was also found that 21% of IL-28A levels was explained from the combination by all ADS dimensions, whereas OPS was the predictor for lower concentrations. The obtained results revealed meaningful correlations between psycho neuro–immunological factors in pathogenesi...
Folia Medica
Minimal residual disease refers to a leukemia cell population that is resistant to chemotherapy o... more Minimal residual disease refers to a leukemia cell population that is resistant to chemotherapy or radiotherapy and leads to disease relapse. The assessment of MRD is crucial for making an accurate prognosis of the disease and for the choice of optimal treatment strategy. Here, we review the advantages and disadvantages of the available genetic and phenotypic methods and focus on the multiparametric flow cytometry as a promising method with greater sensitivity, speed, and standardization options. In addition, we discuss how the application of automated data analysis outweighs the use of complex combinations of windows and gates in classical analysis, thus eliminating subjective evaluation.
Zentralblatt für Pathologie, 1993
The parallel expression of ABH blood-group antigens and of the carcinoembryonic antigen was exami... more The parallel expression of ABH blood-group antigens and of the carcinoembryonic antigen was examined applying the biotin-streptavidin immunostaining system. Monoclonal antibodies to the ABH antigens newly produced by us and a polyclonal antibody to the carcinoembryonic antigen (Ortho) were used as primary antibodies. Human tumours derived from six different organs were studied. ABH antigens showed a heterogeneous expression. They were entirely missing in some neoplastic tissues and found in single or in clustered tumour cells in others. The staining for the carcinoembryonic antigen revealed stronger intensity and covered large malignant areas. The possible functions of ABH blood-group antigens as tumour-associated structures are discussed. A number of tumour-associated antigens have been identified which may serve to improve the early diagnostics of tumour processes in man. Recently, the blood-group antigens (BGA) from the ABO/H system have attracted the attention of many investigat...
American Journal of Reproductive Immunology, 1995
PROBLEM: To test the relative impact of epididymal versus ejaculated sperm in immunologic inferti... more PROBLEM: To test the relative impact of epididymal versus ejaculated sperm in immunologic infertility.METHOD: Human antibody binding to epididymal and ejaculated spermatozoa was compared by flow cytometry (FCM) since it allows quantitative analysis of viable sperm while ignoring nonsperm cells. To select sera for FCM, GAT, TAT, and ELISA were applied on 145 sera from fertile men, idiopathically infertile and varicocele patients.RESULTS: All GAT/TAT‐positive infertile patients, a representative group of varicocele patients and the fertile control, were assessed by FCM. Higher reactivity toward epididymal sperm revealed 18/22 sera while only four out of them bound to ejaculated sperm stronger than the control. All varicocele sera were positive against epididymal while negative against ejaculated spermatozoa.CONCLUSIONS: Epididymal sperm antigens may play a predominant role in some cases of immunologic infertility. Such patients might not be adequately diagnosed and respectively treate...
Biologicals, 1993
The present study assesses the influence of the immunophenotyping procedure, the type of monoclon... more The present study assesses the influence of the immunophenotyping procedure, the type of monoclonal antibody (MAb) and flow cytometer used, upon the reproducibility of the results of estimation of the normal values for CD3, CD4, and CD8 positive lymphocyte subpopulations. A hundred healthy adults were immunophenotyped by direct immunofluorescence with commercially available (OKT3-FITC, OKT4-PE and OKT8-PE), and locally prepared (HL3-FITC, HL4-PE and HL8-PE)MAb. The samples were analysed on FACStar flow cytometer. The maximal values of the analytical (CD3-0.96, CD4-1.05, CD8-1.02), the individual (CD3-2.33, CD4-2.88, CD8-2.86), and the population (CD3-8.7, CD4-8.8, CD8-7.5) variation prove that the precision of immunophenotyping procedure has no effect on the variation of the values of the CD3+, CD4+ and CD8+ subpopulations. The values of all three markers showed Gaussian type of distribution. Regression analysis proved that the values of these subpopulations determined by MAb from two sources were highly correlated (r > 0.9) but the paired t-test gave statistically significant difference in the values of CD8 (1.23%, P > 0.001). Mean, SD and 95% range of the sizes of the positive subsets determined by OKT3, OKT4 and OKT8 showed that the results were very close to the results from a multicenter study of adult Caucasians. The values we find fall within the cited critical range for these markers; so we can conclude that if good care is taken for the precision of the procedure, MAb source and type of flow cytometer do not influence the values of the assayed subsets.
European Journal of Immunology, Apr 1, 1979
In vivo treatment with antineoplastic compounds has been reported to lead to the expression of ne... more In vivo treatment with antineoplastic compounds has been reported to lead to the expression of new antigenic specificities which were not detected on parental cells, and which were transmissible as a genetic character. The current study is concerned with antibody-dependent cellular cytotoxic (ADCC) activity in serum of syngeneic mice challenged with LY/DTIC cells, a subline of LY murine lymphoma, antigenically altered by the drug DTIC. LY/DTIC target cells coated with LY/DTICimmune serum were specifically lysed by virgin lymphocytes. The genetic background of the effector cells, whether syngeneic, allogeneic or xenogeneic, did not produce significant differences in the percentage of target-cell lysis. ADCC activity was reduced when the immune serum was added directly to the incubation medium, without precoating. Although sera from individual animals exhibited different levels of ADCC activity, they nevertheless followed the general trend of the pooled sera. Peak activity of ADCC was obtained in the sera collected on Days 8 and 30 after LY/DTIC cell challenge. The ADCC activity elicited by LY/DTIC cells may contribute to the rejection of drug-altered tumour cells.
PROBLEMS of Infectious and Parasitic Diseases
Background: SARS-CoV-2, the virus responsible for COVID-19 pandemic, has posed huge global health... more Background: SARS-CoV-2, the virus responsible for COVID-19 pandemic, has posed huge global health challenges. Understanding the immune response to SARS-CoV-2 infection, and in particular – the role of B cells in the generation of immune memory is crucial for assessing the durability of protective immunity. Materials and Methods: In this longitudinal prospective study, individuals who had recovered from SARS-CoV-2 infection were included. Peripheral venous blood samples were collected at three time intervals post symptom onset (PSO): 1-3 mo, 4-8 mo, and 9-12 mo. The humoral immune response was evaluated by measuring anti-SARS-CoV-2 IgG, virus-neutralizing antibody activity, total S1-specific B-cells, and B cell subpopulations. Results: The levels of anti-SARS-CoV-2 specific IgG antibodies decreased from 390.3 to 204.5 BAU/ml in the first 6-8 months PSO but did not significantly decrease further until the 12 th mo (126.6 BAU/ml). Virus-neutralizing antibodies (activity decreased by 20...
Folia medica
Human blood group antigens are genetically determined structures of undefined nature found not on... more Human blood group antigens are genetically determined structures of undefined nature found not only in red blood cells but also in various tissues. Monoclonal antibodies against A and B human blood group antigens (B7H4 and D1E8) were obtained. Their tissue compatibility was tested in 11 different normal human organs taken from people with A1 and B blood group phenotypes. The antigen-antibody reaction was visualized using the indirect immunoperoxidase technique. The results of the study showed that the antibodies B7H4 and D1E8 react specifically with the tissue blood group antigens in healthy subjects. The findings obtained in most of the tested organs were in accord with those described by other researchers. Differences were found only in the stomach, urothelium, the lungs and the thyroid gland. A possible explanation of the results is the difference in the epitopes recognized by the monoclonal antibodies used by us and the blood group antibodies applied by other researchers. It is ...
Tackling the World's Fastest-Growing HIV Epidemic: More Efficient HIV Responses in Eastern Europe and Central Asia, 2020
Bulgarian Journal of Agricultural Science, 2000
Zeitschrift für Alternsforschung, 1976
The authors study the morphology of the cells in the peritoneal exsudate and the phosphatase acti... more The authors study the morphology of the cells in the peritoneal exsudate and the phosphatase activity of the peritoneal macrophages, obtained from rats with Selye's Progeria-like syndrome. Judging by their morphologic characteristics the macrophages of the experimentally aged rats, before and after contact with Brucella abortus 19 do not differ from those obtained from the unsubjected to experimental ageing rats. The acid phosphatase activity and the adenosinetriphosphatase activity of the macrophages prior to contact with Brucella abortus 19 are nearly identical for both groups of animals. Following contact the activity of the enzymes increases but this increase is slower by aged rats and reaches its maximum 1--2 days later.
Folia medica, 1994
The human ABH blood-group antigen expression of cytokeratin, S-100 protein, and the melanoma-asso... more The human ABH blood-group antigen expression of cytokeratin, S-100 protein, and the melanoma-associated antigen was studied in 12 cases of pigmented nevi. The biotin-streptavidin immunostaining system was applied. ABH antigens were found both in the endothelial cells and in the germinative epidermal layer. The flat overlaying epithelium, single melanocytes and melanoblasts were positive for cytokeratin. Some basilar cells and the giant multinucleate cells in the Spitz nevi stained for S-100 protein. The melanoma-associated antigen was visualized in the upper epidermal layers as well as in some melanocytes. The present article discusses the specificity of the antigens studied as well as some immunologic traits of potential malignant development of Spitz nevi.