Two large banks are set to continue their participation in the jumbo mortgage-backed security market, including one deal backed largely by loans from banks that have plenty of capacity to hold mortgages in portfolio. JPMorgan Chase is set to issue a $262.23 million jumbo MBS with originations largely sourced from First Republic Bank and Chase. The deal is backed solely by 15-year fixed-rate mortgages, which many banks have been willing to retain in portfolio in recent…