Passage
Proteins known as cancer/testis antigens (CTAs) are normally expressed exclusively in germline cells but may become reactivated in cancer cells due to chromatin remodeling (eg, gene activation due to DNA demethylation) . Because of their normally restricted expression, CTAs can elicit an immune response when expressed by cancer cells derived from other tissue types. Several CTAs are known to inhibit apoptosis or promote cell division in cancer cells, making them more resistant to traditional chemotherapy. Accordingly, cancer research instead has focused on immunotherapy treatments targeting CTAs.In a study, researchers isolated peripheral blood lymphocytes (B lymphocytes, T lymphocytes, natural killer [NK] cells) from two healthy individuals. These lymphocytes were cultured with signaling factors known to promote helper T cell (T_h cell) proliferation. After 7 days, cultures from both individuals were enriched for T_h cells, which expressed proteins generally involved in activating other lymphocytes. A subset of the cultures from each individual was treated with 5-Aza-CdR, a DNA-demethylating agent. Reverse transcription polymerase chain reaction (RT-PCR) was used to analyze expression of several CTAs (ie, MAGEs) in both untreated and 5-Aza-CdR-treated lymphocyte cultures, as well as in untreated melanoma (skin cancer) and breast cancer cell lines (Figure 1) . GAPDH, a protein expressed in most cell types, served as a control.
Figure 1 Gels showing RT-PCR results for MAGE expressionAdditional peripheral blood lymphocytes were isolated from the same healthy individuals and incubated with the T_h cells that had been treated with 5-Aza-CdR. After 11 days, these cultures consisted primarily of an increased number of NK cells and cytotoxic T lymphocytes (CTLs) . Researchers found that these CTLs were able to induce apoptosis in breast cancer cell lines but did not affect healthy cells.Researchers used a similar procedure to induce CTL proliferation in lymphocytes isolated from a breast cancer patient. The CTLs were then incubated with cells obtained from a biopsy of the patient's cancerous tissue. Following this experiment, researchers found that apoptosis was induced in the patient's cancer cells.
Kirkin et al. Adoptive cancer immunotherapy using DNA-demethylated T helper cells as antigen-presenting cells. Nature Communications. 2018; 9:785
-Scientists find that the tumor cells of some breast cancer patients are resistant to CTLs generated using the methods described in the passage. The best explanation for this finding would be:

Question

Passage
Proteins known as cancer/testis antigens (CTAs) are normally expressed exclusively in germline cells but may become reactivated in cancer cells due to chromatin remodeling (eg, gene activation due to DNA demethylation) . Because of their normally restricted expression, CTAs can elicit an immune response when expressed by cancer cells derived from other tissue types. Several CTAs are known to inhibit apoptosis or promote cell division in cancer cells, making them more resistant to traditional chemotherapy. Accordingly, cancer research instead has focused on immunotherapy treatments targeting CTAs.In a study, researchers isolated peripheral blood lymphocytes (B lymphocytes, T lymphocytes, natural killer [NK] cells) from two healthy individuals. These lymphocytes were cultured with signaling factors known to promote helper T cell (T_h cell) proliferation. After 7 days, cultures from both individuals were enriched for T_h cells, which expressed proteins generally involved in activating other lymphocytes. A subset of the cultures from each individual was treated with 5-Aza-CdR, a DNA-demethylating agent. Reverse transcription polymerase chain reaction (RT-PCR) was used to analyze expression of several CTAs (ie, MAGEs) in both untreated and 5-Aza-CdR-treated lymphocyte cultures, as well as in untreated melanoma (skin cancer) and breast cancer cell lines (Figure 1) . GAPDH, a protein expressed in most cell types, served as a control.

Figure 1 Gels showing RT-PCR results for MAGE expressionAdditional peripheral blood lymphocytes were isolated from the same healthy individuals and incubated with the T_h cells that had been treated with 5-Aza-CdR. After 11 days, these cultures consisted primarily of an increased number of NK cells and cytotoxic T lymphocytes (CTLs) . Researchers found that these CTLs were able to induce apoptosis in breast cancer cell lines but did not affect healthy cells.Researchers used a similar procedure to induce CTL proliferation in lymphocytes isolated from a breast cancer patient. The CTLs were then incubated with cells obtained from a biopsy of the patient's cancerous tissue. Following this experiment, researchers found that apoptosis was induced in the patient's cancer cells.
Kirkin et al. Adoptive cancer immunotherapy using DNA-demethylated T helper cells as antigen-presenting cells. Nature Communications. 2018; 9:785
-Scientists find that the tumor cells of some breast cancer patients are resistant to CTLs generated using the methods described in the passage. The best explanation for this finding would be:

Quizplus · Accepted Answer

11ecba2d_103f_6f81_a3bf_a92d14287a22_MD0008_00 The adaptive immune system comprises both T lymphocytes and B lymphocytes, each of which recognize specific foreign antigens and mount specialized immune responses against pathogens. As the passage states, immune cells are also able to recognize cancer/testis antigens (CTAs), proteins normally expressed exclusively in germline cells that become reactivated in cancer cells. Cytotoxic T lymphocytes (CTLs) recognize foreign antigens displayed by specialized surface proteins on pathogen-infected or cancerous cells. These cell surface antigen-presenting proteins, known as major histocompatibility (MHC) proteins, display fragments of proteins present within a given cell. Accordingly, CTA-expressing cancer cells would display CTA fragments on their MHC proteins. Upon binding foreign antigens (eg, CTAs) displayed by MHCs, CTLs release cytotoxins that induce apoptosis of diseased cells. In the passage experiment, researchers cultured CTLs that could recognize CTAs and induce apoptosis in breast cancer cells. As the question states, tumor cells of some breast cancer patients are resistant to these CTLs. To induce apoptosis in CTA-expressing cancer cells, CTLs mustbind CTAs displayed by cancer cell MHC proteins. Therefore, a lack of CTA-presenting MHC proteins on tumor cells would prevent CTLs from binding, making these tumor cells CTL-resistant. (Choice A) Antibodies bind foreign antigens, marking them for destruction by immune cells. Accordingly, enhanced antibody binding to tumor cells would likely increase the ability of certain immune cells to respond to these cancer cells and would not explain CTL resistance. (Choice B) Increased CTA expression would likely increase the number of CTAs displayed by MHC proteins of tumor cells, enhancing the ability of CTLs to recognize CTAs and induce tumor cell apoptosis. (Choice C) Self-antigens are peptides expressed by healthy cells. Innate immune cells (ie, natural killer cells) may recognize diseased cells with reduced numbers of self-antigens displayed by MHC proteins. Therefore, impaired self-antigen expression would likely make tumor cells more susceptible to innate immune cells. In addition, the ability of CTLs to induce apoptosis in cancer cells would not be affected by changes in self-antigen expression because CTLs bind foreign (non-self) antigens. Educational objective: Cytotoxic T lymphocytes bind foreign antigens presented by cell surface antigen-presenting proteins, called major histocompatibility (MHC) proteins. Upon binding foreign antigens, CTLs release cytotoxins that induce apoptosis in infected cells.

Passage Proteins Known as Cancer/testis Antigens (CTAs) Are Normally Expressed Exclusively